Methods And Devices For Sequencing Nucleic Acids In Smaller Batches

ABSTRACT

The invention provides methods and compositions, including, without limitation, algorithms, computer readable media, computer programs, apparatus, and systems for determining the identity of nucleic acids in nucleotide sequences using, for example, data obtained from sequencing by synthesis methods. A plurality of smaller flow cells is employed, each with a relatively small area to be imaged, in order to provide greater flexibility and efficiency.

FIELD OF THE INVENTION

The invention relates to methods, compositions, devices, systems andkits including, without limitation, reagents, mixtures, data processingsteps, algorithms, computer readable media, and computer programs, fordetermining the identity of nucleic acids in nucleotide sequences using,for example, data obtained from sequencing by synthesis, sequencing byligation and other nucleic acid sequencing methods. In one embodiment,the present invention provides methods and devices for smallersequencing projects, i.e. sequencing nucleic acids in smaller batches.

BACKGROUND OF THE INVENTION

Over the past 25 years, the amount of DNA sequence information that hasbeen generated and deposited into Genbank has grown exponentially. Manyof the next-generation sequencing technologies use a form of sequencingby synthesis (SBS), wherein specially designed nucleotides and DNApolymerases are used to read the sequence of chip-bound, single-strandedDNA templates in a controlled manner. Other next-generation sequencingtechnologies may use native nucleotides and/or polymerases or labeledoligonucleotides and ligation enzymes to determine nucleic acidsequences. To attain high throughput, many millions of such templatespots, each being either single or multiple molecules, are arrayedacross a sequencing chip and their sequence is independently read outand recorded. The desire to perform high throughput sequencing stemsfrom the need for faster processing and reduced costs. However,commercial high throughput systems, while reducing the cost of largescale sequencing (e.g. 10-100 gigabases), make smaller scale sequencing(e.g. 100 megabases to 1 gigabases) costly and inconvenient. There is,therefore, a continued need for improved methods and devices forsequencing nucleic acid in order to address the practical day-to-daysequencing work of the average scientist.

SUMMARY OF THE INVENTION

At present, all highly parallel commercial sequencers require the userto run an entire, large flow cell, no matter what size the study or howmany (or few) separate samples one might need. This is due to currentdesigns which employ a single flow cell or dual flow cells, wherein allreagents and washes occur at a single station for each flow cell andvalving is used to change the reagent delivered to the flow cell. Thislack of flexibility has a number of unfavorable consequences. First, itmeans that one must either tolerate the waste and higher costs ofsmaller sequencing jobs (e.g. 100 megabases to 1 gigabases), or waituntil a number of smaller sequencing jobs can be combined into a largerrun. Second, it means that large sequencing projects will be favored(e.g. at a core facility) since they are more cost-effective, causingthe average researcher with the smaller project to be excluded or (atbest) forced to wait days to weeks before getting access to a sharedmachine.

In one embodiment, the present invention provides important features ina sequencing system that result in greater flexibility. That is to say,researchers who want to run a relatively small sequencing study on asingle sample, or parallel studies on multiple samples (say sequence thesame five genes from 10 individuals), can do so without waiting days toweeks to be “fit” into a larger run. Indeed, researchers can choose thesize of their projects and scale their reagent usage accordingly,resulting in cost-effective smaller runs.

In one embodiment, this flexibility is provided by a) scaling down thesize of the reaction chamber or flow cell to where the dimensions of thechip or the area to be imaged are less than the size of a standardmicroscope slide (i.e. less than 75 mm×25 mm) and preferablyconsiderably less (e.g. 35 mm×2.5 mm, or smaller), and/or b) providing aplurality of these smaller flow cells (typically, 2, 4, 8, 10, 16 oreven 20 flow cells) that are (preferably) both moveable and removeable.By scaling down the size of the flow cell, reagent volumes are reduced(e.g. to between 10 and 40 microliters, and more typically toapproximately 20 microliters) in order to reduce costs. In addition, theamount of area that is imaged is reduced (allowing for faster imagingtimes and less expensive imaging equipment). Thus, rather thancontinuing to scale up sequencing, the present invention, in oneembodiment, takes the approach of sequencing in smaller batches.

These changes also provide advantages in the context of genomic analysisand diagnostic testing, including but not limited to, sequencing ofpolymorphic areas in the genome that are linked to disease. Rather thanusing a relatively large single array or chip in a single flow cell,wherein the nucleic acid of a plurality of subjects, including but notlimited to human patients, are combined, the present inventioncontemplates, in one embodiment, using a plurality of smaller arrays orchips in smaller flow cells, wherein any one chip contains the nucleicacid of but a single subject, including but not limited to a singlehuman patient. In another embodiment, multiple subjects are sequenced ona single flow cell, but the sequencing may be completed faster and morecost effectively as compared to larger flow cell systems. Of course, itis not intended that the present invention be limited to sequencingusing arrays or chips. Nucleic acid to be sequenced can be housed in theflow cell in a number of ways, including but not limited to, immobilizedon beads where the beads are in fixed positions or are not in fixedpositions (e.g. free to move about) within the flow cell.

In one embodiment, the present invention contemplates a method forcarrying out steps of a nucleic acid sequencing protocol (e.g. where theprotocol involves numerous cycles, each cycle having a number of steps),at least one step performed first in a cycle, some steps performed laterin said cycle, said method comprising: a) providing: i) a camera; ii) aplurality of reservoirs, at least one of which comprising nucleic acidsequencing reagents, and at least one of which comprising wash buffer;iii) first nucleic acids in a first flow cell and second nucleic acidsin a second flow cell, said first and second nucleic acids comprisingtemplates to be sequenced in an area to be imaged, said flow cellsmounted on iv) a moveable support, said support configured to move saidflow cells to v) a plurality of stations, at least some of which arereagent delivery stations, each reagent delivery station in fluidiccommunication with at least one of said reservoirs and associated withat least one step, or portion thereof, of said nucleic acid sequencingprotocol, at least one of said steps comprising positioning a flow cellsuch that said area to be imaged can be imaged by said camera; b) movingsaid moveable support under conditions such that said first flow cell isaligned with and connected to the reagent delivery station associatedwith a first step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a first reservoir of saidplurality of reservoirs; c) introducing reagent via one of saidreservoirs into said first flow cell under conditions such that saidreagent contacts said first nucleic acids; and d) moving said moveablesupport'under conditions such that i) said first flow cell isdisconnected with said reagent delivery station associated with saidfirst step and aligned with a station associated with a later step insaid sequencing protocol, and ii) said second flow cell is aligned withand connected to said reagent delivery station associated with saidfirst step in said sequencing protocol, whereby said second flow cell isin fluid communication with at least said first reservoir of saidplurality of reservoirs. In one embodiment, the method further comprisesperforming a plurality of cycles, wherein steps b, c and d are repeated.In one embodiment, said later step comprises the second step in saidsequencing protocol and said first flow cell is aligned with andconnected to said reagent delivery station associated with said secondstep in said sequencing protocol, whereby said first flow cell is influid communication with at least said second reservoir of saidplurality of reservoirs. In one embodiment, said reagent deliverystation associated with a first step in said sequencing protocol alsoperforms the second step in said sequencing protocol. In one embodiment,said later step comprises the third step in said sequencing protocol. Inone embodiment, said second step is a wash step wherein said firstnucleic acids of said first flow cell is contacted with wash buffer. Inone embodiment, said at least one station is a dwell station and not areagent delivery station. In one embodiment, said later step comprisespausing at said dwell station. In one embodiment, one of said reservoirscomprises labeled nucleotide analogues comprising a base and a sugar. Inone embodiment, said sugar is an acyclic sugar. In one embodiment, thelabeled nucleotide analogues comprise a label attached to the base ofsaid nucleotide analogues via a cleavable linker. In one embodiment,said labeled nucleotide analogues further comprising a cleavableblocking group on the sugar. In one embodiment, said labeled nucleotidesare introduced at step c). In one embodiment, the labeled nucleotideanalogues produce the signals required for imaging and sequencing. Inone embodiment, said reagent is introduced into said first flow cell atstep c) in a volume that is less than 50 microliters. In one embodiment,said reagent is introduced into said first flow cell at step c) in avolume of approximately 20 microliters or less (but not less than 5microliters). In one embodiment, the method further comprises the stepe) introducing reagent via said first reservoir into said second flowcell under conditions such that said reagent contacts said secondnucleic acids. In one embodiment, the method further comprises the stepf) moving said moveable support under conditions that i) said first flowcell is disconnected with said reagent delivery station associated withsaid second step and aligned with and connected to the reagent deliverystation associated with a third step in said sequencing protocol,whereby said first flow cell is in fluid communication with at least athird reservoir of said plurality of reservoirs, and ii) said secondflow cell is disconnected with said reagent delivery station associatedwith said first step and aligned with and connected to the reagentdelivery station associated with a second step in said sequencingprotocol, whereby said second flow cell is in fluid communication withat least a second reservoir of said plurality of reservoirs. In oneembodiment, said third step in said sequencing protocol is a wash stepwherein said first nucleic acids of said first flow cell are contactedwith wash buffer. In one embodiment, said first nucleic acids in saidfirst flow cell, are scanned and imaged with said camera after step f).In one embodiment, said first flow cell is removed from said moveablesupport prior to said imaging. In one embodiment, an oxygen scavenger isintroduced into said first flow cell before, during or after said firstnucleic acids are scanned and imaged. In one embodiment, the methodfurther comprises moving said carousel under conditions that said firstflow cell is aligned with and connected to the reagent delivery stationassociated with a fourth step in said sequencing protocol, whereby saidfirst flow cell is in fluid communication with at least a fourthreservoir of said plurality of reservoirs. In one embodiment, saidfourth step in said sequencing protocol is a cleavage step wherein saidfirst chip of said first flow cell is contacted with buffer comprising acleaving agent (which can be an agent that cleaves chemically or anagent that cleaves enzymatically) which cleaves said cleavable linkerunder first conditions. In one embodiment, said cleavable blocking groupis thereafter cleaved under second conditions. In one embodiment, saidfirst conditions and second conditions are different. In one embodiment,the method further comprises moving said carousel under conditions thatsaid first flow cell is disconnected with said reagent delivery stationassociated with said fourth step and aligned with and connected to thereagent delivery station associated with a fifth step in said sequencingprotocol, whereby said first flow cell is in fluid communication with atleast a fifth reservoir of said plurality of reservoirs. In oneembodiment, said fifth step in said sequencing protocol comprisingintroducing a cleaving agent scavenger to address (e.g. neutralize,bind, or otherwise render inactive) leftover cleaving agent, which mightprematurely cleave in the next cycle (e.g. cause an unintended andundesired cleavage of a label from a nucleotide, before the nucleotideis incorporated, or before the label can be detected). In oneembodiment, said first nucleic acids are on a first chip in said firstflow cell, and said chip is scanned and imaged with said camera afterstep f). In one embodiment, said first flow cell is removed from saidmoveable support prior to said imaging. In one embodiment, said firstflow cell is aligned with and connected to the reagent delivery stationassociated with a fourth step in said sequencing protocol prior to saidimaging with said camera. In one embodiment, said second nucleic acidsare on a second chip in said second flow cell, and said second chip isscanned and imaged after said first chip in said first flow cell isscanned and imaged. In one embodiment, said instrument comprises asecond camera, and said second chip in said second flow cell is scannedand imaged with said second camera at the same time said first chip insaid first flow cell is scanned and imaged with said first camera. Inone embodiment, said instrument comprises a second camera (FIG. 64A),and said first chip in said first flow cell is simultaneously scannedand imaged with said first and second cameras. In one embodiment, athird flow cell comprising third nucleic acids is added to said moveablesupport before said first flow cell is scanned and imaged with saidcamera. In one embodiment, a third flow cell comprising third nucleicacids is present on said moveable support when said first flow cell isscanned and imaged with said camera. In one embodiment, said third flowcell was not present on said moveable support prior to step b). In oneembodiment, a third flow cell comprising third nucleic acids is added tosaid moveable support after said first flow cell is scanned and imagedwith said camera. In one embodiment, said area to be imaged on saidfirst and second chips is (on each) approximately 35 mm×2.5 mm, or less(but not less than 10% of this area). In one embodiment, said first chipremains in said first flow cell while it is imaged. at least a portionof said first and second flow cells is transparent. In one embodiment,said first flow cell is in a first position on said moveable support andsaid second flow cell is in a second position on said moveable support,said first and second positions being in a fixed relationship on saidmoveable support. In one embodiment, said first flow cell is removedfrom said first position on said moveable support and repositioned in anew position on said moveable support, said new position being differentfrom said first or second position. In one embodiment, said first flowcell is removed from said first position before or after a first cycleof steps is completed. In one embodiment, said moveable support is acarousel. In one embodiment, said carousel is under the control of (e.g.in electronic communication with) a processor. In one embodiment, saidprocessor can be programmed to change the movement of said carousel. Inone embodiment, said first nucleic acids are immobilized on a pluralityof beads in said first flow cell, and said beads are scanned and imagedwith said camera after step f). In one embodiment, said first nucleicacids are from a first subject and said second nucleic acids are from asecond subject.

In one embodiment, said first nucleic acids are from a first subject,said second nucleic acids are from a second subject, and said thirdnucleic acids are from a third subject. In one embodiment, the presentinvention contemplates a system comprising i) a plurality of reservoirs,at least one of which comprises nucleic acid sequencing reagents; ii)first nucleic acids in a first flow cell and iii) second nucleic acidsin a second flow cell, said first and second nucleic acids comprisingtemplates to be sequenced in an area to be imaged, said flow cellsmounted on iv) a moveable support, said support configured to move saidflow cells to v) a plurality of reagent delivery stations, each stationin fluidic communication with at least one of said reservoirs andassociated with at least one step, or portion thereof, of said nucleicacid sequencing protocol, said steps of said protocol carried out by vi)a programmable processor. In one embodiment, said first nucleic acidsare from a first subject and said second nucleic acids are from a secondsubject. In one embodiment, the system further comprises a camera forimaging one or more flow cells. In one embodiment, at least one of saidstations is positioned such that said area to be imaged can be imaged bysaid camera. In one embodiment, said system is contained within thehousing of an instrument, said instrument comprising a user interface,said user interface in (electronic) communication with said programmableprocessor. In one embodiment, said system further comprises an imagingplatform capable of x/y movement, said imaging platform positioned undersaid camera. further comprising a transfer means capable of moving atleast one of said flow cells from said moveable support to said imagingplatform. In one embodiment, said area to be imaged in said first andsecond flow cells is (on each) approximately 35 mm×2.5 mm, or less (butnot less than 10% of this area). In one embodiment, said templates to besequenced are on chips in said flow cells. In one embodiment, saidtemplates are on beads in said flow cells. In one embodiment, said beadsare immobilized. In one embodiment, said beads are immobilized byattachment of each bead to a solid support or to the flow cell itself.

In one embodiment, the present invention contemplates a method forcarrying out steps of a nucleic acid sequencing protocol (e.g. where theprotocol involves numerous cycles, each cycle having a number of steps),at least one step performed first in a cycle, some steps performed laterin said cycle, said method comprising: a) providing: i) an imagingplatform capable of x/y movement; ii) a camera position above saidimaging platform; iii) a plurality of reservoirs, at least one of whichcomprising nucleic acid sequencing reagents, and at least one of whichcomprising wash buffer; iv) a first chip (or solid support) in a firstflow cell and a second chip (or solid support) in a second flow cell,said first and second chips comprising nucleic acid templates to besequenced in an area to be imaged, said flow cells mounted on v) acarousel, said carousel configured to move said flow cells to vi) aplurality of stations, at least some stations comprising reagentdelivery stations in fluidic communication with at least one of saidreservoirs and associated with at least one step, or portion thereof, ofsaid nucleic acid sequencing protocol; and vii) a transfer means capableof moving at least one of said flow cells from said carousel to saidimaging platform, and from said imaging platform to said carousel; b)moving said carousel under conditions such that said first flow cell isaligned with and connected to the reagent delivery station associatedwith a first step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a first reservoir of saidplurality of reservoirs; c) introducing reagent via one of saidreservoirs into said first flow cell under conditions such that saidreagent contacts said nucleic acid templates of said first chip; and d)moving said carousel under conditions such that i) said first flow cellis disconnected with said reagent delivery station associated with saidfirst step and aligned with a station associated with a later step insaid sequencing protocol, and ii) said second flow cell is aligned withand connected to said reagent delivery station associated with saidfirst step in said sequencing protocol, whereby said second flow cell isin fluid communication with at least said first reservoir of saidplurality of reservoirs. In one embodiment, the method further comprisesperforming a plurality of cycles, wherein steps b, c and d are repeated.In one embodiment, said later step comprises the second step in saidsequencing protocol and said first flow cell is aligned with andconnected to said reagent delivery station associated with said secondstep in said sequencing protocol, whereby said first flow cell is influid communication with at least said second reservoir of saidplurality of reservoirs. In one embodiment, said reagent deliverystation associated with a first step in said sequencing protocol alsoperforms the second step in said sequencing protocol. In one embodiment,said later step comprises the third step in said sequencing protocol. Inone embodiment, said second step is a wash step wherein said first chipof said first flow cell is contacted with wash buffer. In oneembodiment, at least one station is a dwell station and not a reagentdelivery station. In one embodiment, said later step comprises pausingat said dwell station. In one embodiment, one of said reservoirscomprises labeled nucleotide analogues comprising a base and a sugar. Inone embodiment, said sugar is an acyclic sugar. In one embodiment,labeled nucleotide analogues comprise a label attached to the base ofsaid nucleotide analogues via a cleavable linker. In one embodiment,said labeled nucleotide analogues further comprising a cleavableblocking group on the sugar. In one embodiment, said labeled nucleotidesare introduced at step c). In one embodiment, the labeled nucleotideanalogues produce the signals required for imaging and sequencing. Inone embodiment, the method further comprises the step e) introducingreagent via said first reservoir into said second flow cell underconditions such that said reagent contacts said nucleic acid templatesof said second chip. further comprising the step f) moving said carouselunder conditions that i) said first flow cell is disconnected with saidreagent delivery station associated with said second step and alignedwith and connected to the reagent delivery station associated with athird step in said sequencing protocol, whereby said first flow cell isin fluid communication with at least a third reservoir of said pluralityof reservoirs, and ii) said second flow cell is disconnected with saidreagent delivery station associated with said first step and alignedwith and connected to the reagent delivery station associated with asecond step in said sequencing protocol, whereby said second flow cellis in fluid communication with at least a second reservoir of saidplurality of reservoirs. said third step in said sequencing protocol isa wash step wherein said first chip of said first flow cell is contactedwith wash buffer. In one embodiment, after step f), said first chip insaid first flow cell is moved to a station that is not in fluidcommunication with a reservoir. In one embodiment, said first flow cellis transferred from said station not in fluid communication with areservoir to said imaging platform via said transfer means. In oneembodiment, said flow cell transferred to said imaging platform isscanned and imaged with said camera. In one embodiment, an oxygenscavenger is introduced into said first flow cell before, during orafter said flow cell is scanned and imaged. In one embodiment, aftersaid flow cell is imaged, said flow cell is transferred back to saidcarousel via said transfer means. In one embodiment, said flow cell istransferred back to the same station from which it was transferred. Inone embodiment, said flow cell is transferred back to a differentstation. In one embodiment, the method further comprises moving saidcarousel under conditions that said first flow cell is aligned with andconnected to the reagent delivery station associated with a fourth stepin said sequencing protocol, whereby said first flow cell is in fluidcommunication with at least a fourth reservoir of said plurality ofreservoirs. said fourth step in said sequencing protocol is a cleavagestep wherein said first chip of said first flow cell is contacted withbuffer comprising a cleaving agent (which can cleave chemically or,alternatively, enzymatically) which cleaves said cleavable linker underfirst conditions. In one embodiment, said cleavable blocking group isthereafter cleaved under second conditions. In one embodiment, saidfirst conditions and second conditions are different. In one embodiment,the method further comprises moving said carousel under conditions thatsaid first flow cell is disconnected with said reagent delivery stationassociated with said fourth step and aligned with and connected to thereagent delivery station associated with a fifth step in said sequencingprotocol, whereby said first flow cell is in fluid communication with atleast a fifth reservoir of said plurality of reservoirs. In oneembodiment, said fifth step in said sequencing protocol comprisesintroducing a cleaving agent scavenger to address (e.g. neutralize, bindto, or otherwise render inactive) leftover cleaving agent which mightprematurely cleave in the next cycle. In one embodiment, said differentstation is adjacent a reagent delivery station associated with a laterstep in the same cycle of said sequencing protocol. In one embodiment,said different station is the reagent delivery station associated with alater step in the same cycle of said sequencing protocol. In oneembodiment, said different station is adjacent a reagent deliverystation associated with a first step in a later cycle of said sequencingprotocol. In one embodiment, said different station is the reagentdelivery station associated with a first step in a later cycle of saidsequencing protocol. In one embodiment, the method further comprisesmoving said carousel under conditions such that said first flow cell isaligned with and connected to said reagent delivery station associatedwith a first step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a first reservoir of saidplurality of reservoirs, such that a second cycle of said sequencingprotocol can commence. In one embodiment, there is at least one dwellstation between the station from which the flow cell was transferred andthe station adjacent a reagent delivery station associated with a firststep in said sequencing protocol. In one embodiment, said first flowcell is in a first position on said moveable support and said secondflow cell is in a second position on said moveable support, said firstand second positions being in a fixed relationship on said moveablesupport. In one embodiment, said first flow cell is removed from saidfirst position on said moveable support and repositioned in a newposition on said moveable support, said new position being differentfrom said first or second position. In one embodiment, said first flowcell is removed from said first position before or after a first cycleof steps is completed. In one embodiment, said nucleic acid templates insaid first flow cell are from a first human patient and said nucleicacid templates in said second flow cell are from a second human patient.In one embodiment, there is a third flow cell. In one embodiment, saidnucleic acid templates in said first flow cell are from a first humanpatient, said nucleic acid templates in said second flow cell are from asecond human patient, and said nucleic acid templates in said third flowcell are from a third human patient (i.e. each flow cell is limited tothe nucleic acid from at most one patient). In other embodiments, eachflow cell comprises the nucleic acid from at least two patients. In oneembodiment, said area to be imaged in said first and second flow cellsis (on each) approximately 35 mm×2.5 mm, or less (but not less than 10%of this area).

In yet another embodiment, the present invention contemplates a methodfor carrying out steps of a nucleic acid sequencing protocol (e.g. wherea plurality of cycles are performed), at least one step performed firstin a cycle, some steps performed later in said cycle, said methodcomprising: a) providing: i) a plurality of flow cells, the number offlow cells corresponding approximately to (or corresponding exactly to)the number of steps in one cycle, said flow cells comprising nucleicacid templates to be sequenced in an area to be imaged, said flow cellsmounted on ii) a moveable support, said support configured to move saidflow cells to iii) a plurality of stations, at least some of which arereagent delivery stations, each reagent delivery station in fluidiccommunication with at least one reservoir of a plurality of reservoirsand associated with at least one step, or portion thereof, of saidnucleic acid sequencing protocol, some of said reservoirs comprisingreagent; b) moving said moveable support under conditions such that afirst flow cell of said plurality of flow cells is aligned with andconnected to the reagent delivery station associated with a first stepin said sequencing protocol, whereby said first flow cell is in fluidcommunication with at least a first reservoir of said plurality ofreservoirs; c) introducing reagent via one of said reservoirs into saidfirst flow cell under conditions such that said reagent contacts saidnucleic acid templates in said first flow cell; and d) moving saidmoveable support under conditions such that said first flow cell isdisconnected with said reagent delivery station associated with saidfirst step and aligned with a station associated with a later step insaid sequencing protocol. In one embodiment, each cycle comprises fivesteps and the number of flow cells is between five and seven. In oneembodiment, each cycle comprises six steps and the number of flow cellsis between four and eight. In one embodiment, each cycle comprises sevensteps and the number of flow cells is between five and nine. In oneembodiment, the number of flow cells is equal to the number of steps inone cycle. In one embodiment, said nucleic acid templates are on a firstchip in said first flow cell. In one embodiment, said nucleic acidtemplates are immobilized on a plurality of beads in said first flowcell. In one embodiment, said area to be imaged is approximately 35mm×2.5 mm, or less. In one embodiment, said area to be imaged is notless than 10% of the area defined by 35 mm×2.5 mm. In one embodiment,said reagent of step c) comprises labeled nucleotide analogues, saidanalogues comprising i) a label attached to the base of said nucleotideanalogues via a cleavable linker and ii) a cleavable blocking group onthe sugar. In one embodiment, the method further comprises performing aplurality of cycles, wherein steps b, c and d are repeated.

In yet another embodiment, the present invention contemplates a methodfor carrying out steps of a nucleic acid sequencing protocol (e.g. wherea plurality of cycles are performed), at least one step performed firstin a cycle, some steps performed later in said cycle, said methodcomprising: a) providing: i) a plurality of flow cells, said flow cellscomprising nucleic acid templates to be sequenced in an area to beimaged, said flow cells mounted on ii) a moveable support, said supportconfigured to move said flow cells to iii) a plurality of stations, atleast some of which are reagent delivery stations, each reagent deliverystation in fluidic communication with at least one reservoir of aplurality of reservoirs and associated with at least one step, orportion thereof, of said nucleic acid sequencing protocol, some of saidreservoirs comprising reagent; b) positioning (or placing) a first flowcell of said plurality of flow cells on said moveable support such thatit is aligned with and connected to a reagent delivery stationassociated with a first step in said sequencing protocol, whereby saidfirst flow cell is in fluid communication with at least a firstreservoir of said plurality of reservoirs; c) introducing reagent viaone of said reservoirs into said first flow cell under conditions suchthat said reagent contacts said nucleic acid templates in said firstflow cell; and d) moving said moveable support under conditions suchthat said first flow cell is disconnected with said reagent deliverystation associated with said first step and aligned with a stationassociated with a later step in said sequencing protocol. In oneembodiment, the method further comprises removing said first flow cellfrom said moveable support. In one embodiment, the method furthercomprises adding one or more additional flow cells to said moveablesupport.

While the preferred mode of sequencing is sequencing by synthesis, manyof the features and advantages described herein can be applied to othersequencing approaches. For example, a plurality of smaller flow cellscan be used with the pyrosequencing approach that employs four natural(native) nucleotides (comprising a base of adenine (A), cytosine (C),guanine (G), or thymine (T)) and several other enzymes for sequencingDNA. In this approach, the detection is based on the pyrophosphate (PPi)released during the DNA polymerase reaction, the quantitative conversionof pyrophosphate to adenosine triphosphate (ATP) by sulfurylase, and thesubsequent production of visible light by firefly luciferase. Similarly,a plurality of smaller flow cells can be used with either sequencing byhybridization or sequencing by ligation. Sequencing by hybridization iswidely used in several different forms, including dynamic hybridization,and is the primary method currently used in all microarray detectionformats. Sequencing by ligation is based on the ability of DNA ligase tojoin the ends of two oligonucleotides annealed next to each other on atemplate. For example, two oligonucleotides can be designed to anneal toboth sides of a SNP site, and by detecting the formation of ligationproduct, the genotype of a target can be inferred. Finally, still otherapproaches to sequencing are contemplated, such as sequencing bydegradation, single molecule sequencing, and nanopore (luminescent,fluorescent, FRET or current/voltage readout or combinations thereof).

In one embodiment, the flow cells do not move, but “stations” with therequisite reagent (or wash) delivery connections and controls (e.g.tubing, channels, inlets, ports, pumps, valving, etc.) move to the flowcells. However, in a preferred embodiment, the flow cells are inpositions on a moving support and the support moves the flow cells todiscrete locations, some locations having stations, some stationscomprising the requisite reagent delivery connections and controls (inthis embodiment, imaging can be done where the flow cells are moved tothe camera, or the camera is moved to the flow cells, or both flow cellsand camera move). In a preferred embodiment, a plurality of smaller flowcells are mounted on a moving plate (or other support) that brings eachflow cell to a “station” associated with at least one step (or partialstep) in the sequencing protocol (e.g. reagent introduction, washing,etc.). In some embodiments, two or more steps are done at a singlestation. In some embodiments, a portion of a step is done at one stationand the remaining portion of the step is done at another station.Movement from station to station requires, in many cases, disconnectingfrom one set of ports and connecting to another set of ports (describedmore fully below with regard to the figures).

Thus, in one embodiment, the present invention contemplates a method forcarrying out steps of a nucleic acid sequencing protocol, comprising: a)providing a first chip in a first flow cell, a second chip in a secondflow cell, nucleic acid to be sequenced, nucleic acid sequencingreagents, and a first camera; and b) introducing said nucleic acid to besequenced and said nucleic acid sequencing reagents into said first andsecond flow cells under conditions such that, while said first chip insaid first flow cell is undergoing one or more reaction steps, saidsecond chip in a second flow cell is being scanned and imaged with saidfirst camera. In one embodiment, the method further comprises the stepc) wherein said first chip in said first flow cell is scanned and imagedwith said first camera after step b). In one embodiment, the methodfurther comprises the step c) wherein said first chip in said first flowcell is scanned and imaged with a second camera after step b). In oneembodiment, said nucleic acid sequencing reagents comprise labelednucleotide analogues and polymerase. In one embodiment, said nucleicacid sequencing reagents are introduced into said flow cells in a volumethat is less than 50 microliters. In a preferred embodiment, saidnucleic acid sequencing reagents are introduced into said flow cells ina volume of approximately 20 microliters or less. The first and secondchips typically contain nucleic acid templates derived from genomic DNA(either self-priming hairpins or single-stranded to whicholigonucleotide primers can be hybridized and extended). In oneembodiment, said first and second chips comprise arrays of such nucleicacid in the form of “spots.” In one embodiment, the nucleic acidsequencing reagents pass the spots and produce the signals required forsequencing. In a preferred embodiment, the second chip remains in theflow cell while it is imaged. In a particularly preferred embodiment,said first and second flow cells are positioned on a moving support(such as a rotary stage or carousel). In a preferred embodiment, saidfirst and second flow cells are transparent (or at least a portionthereof is transparent). In a preferred embodiment, said flow cells areincorporated within an instrument (having a housing, within which is thecamera and reservoirs for the various reagents).

The advantages of associating each station with at least one step (orpartial step) in the sequencing protocol are significant. In currentlarge systems, where all steps are done using a single flow cell, oneparticular step may be the “rate limiter.” For example, in current largesystems, the area to be imaged may be so great that the imaging steptakes twice as long (or more) as any other step in the sequencingprotocol. In these systems, subsequent steps are “held up” or delayeduntil the longer step of imaging is complete. By contrast, using aplurality of smaller flow cells at stations associated with a step (orpartial step) in the sequencing protocol, allows one to a) reduce thetime necessary for imaging, and thereby b) reduce the overall processingtime. For example, a simplified sequencing protocol might employ fivesteps (after which the cycle begins again): 1) incorporating nucleotideanalogues with a polymerase; 2) washing; 3) detecting the label of theincorporated nucleotide analogues (imaging); 4) removing the chemicalmoiety of the incorporated nucleotide analogue capping the 3′-OH groupwith a cleaving agent; and 5) washing. If each step (other than theimaging step) takes seven minutes, and the imaging step takes fifteenminutes, using current approaches results in an overall processing timeof forty-three minutes. Using the approach of the present invention, thesmaller flow cell may require only seven minutes to image, reducing theoverall processing time to thirty-five minutes, and—mostimportantly—allowing other flow cells to move to the next station to beprocessed (including moving the next flow cell to the imaging station).Another approach to handling a “rate limiting” step in the sequencingprotocol is to utilize two stations to complete the single step (i.e.each station does a part of the step).

Scaling down the flow cell into multiple flow cells with smaller imagingareas in the manner described herein, while not necessarily resulting inmore throughput (i.e. bases sequenced per day), can increase the speedof each cycle of the sequencing protocol. This is particularly truewhere the number of flow cells corresponds to (or correspondsapproximately to) the number of steps in the sequencing protocol, e.g.the number of steps in one cycle (as contemplated in preferredembodiments) where the protocol typically involves numerous cycles (eg.20 to 100 or more). FIG. 60 and the associated table illustrate theinteresting and subtle dynamics of this scaling down process byquantitatively comparing a single flow cell system, a dual flow cellsystem and an eight flow-cell system (each flow cell containing nucleicacid to be sequenced). In this comparison, the sequencing protocolinvolves seven chemistry steps, approximately equal in time, requiredfor sequencing, followed by the imaging step (for a total of eight stepsin a “cycle”). Each flow cell has a “chip” comprising nucleic acid to besequenced, which is preferably arrayed so that sequencing goes on inparallel in each flow cell—each cycle resulting in a single nucleotidebeing incorporated and detected at each position in the array, the totalnumber of bases being proportional to the imaging area, depending on thearray density.

In the case of a single flow cell, the imaging step involves scanningacross the entire area of the flow cell, such area equal to A. Duringthe imaging, all other steps must wait. Similarly, during the othersteps (i.e. the chemistry steps), there is no imaging. Thus, the imagingsystem is intermittently idle (in this case for seven chemistry steps)and not fully utilized. FIG. 61 shows 4 cycles are completed in arepresentative time period or “T.” Since each cycle results in thesequencing of a single nucleotide at each position in the array, 4cycles translates into 4 bases sequenced per T multiplied by the numberof positions on the array (for shorthand, 4 cycles translates to “4A”bases sequenced). Thus, the time required to sequence 80 bases at eachposition on the array within the single flow cell would be 20T (seeassociated table).

By contrast, in the dual flow cell system, the imaging portion is morefully utilized since the second flow cell may be imaged while the firstis undergoing the seven chemistry steps and vice versa. Interestingly,Table 1 shows that, during the time period (T), the number of totalbases sequenced increases almost two-fold (expressed in the associatedtable as 7A). And yet, the time needed to sequence 80 bases at eachposition in the arrays of both flow cells is not decreased. Thisillustrates that just adding flow cells, particularly flow cells of thesame size, will not maximize efficiency (e.g. will not minimize cycletime). Furthermore, it should be stressed that, in this example, thenumber of flow cells (two) does not correspond (even approximately) tothe number of sequencing steps (8 steps total). By corresponding orapproximately corresponding (i.e. plus or minus two) to the number ofsteps, the maximum or near maximum efficiencies can be obtained.

In the eight flow cell configuration, each of the flow cells is 1/7A inarea and therefore takes 1/7^(th) the time to image. Moreover, the eightflow cells correspond to the number of sequencing steps (which is apreferred embodiment). In this configuration, the imaging station isstill fully utilized and therefore produces the same amount of sequenceinformation over a given time period as the dual flow cell configuration(expressed in the table 1 as 7A); however, since it has a greater numberof fluid delivery stations, it can complete more cycles in a given timethan either the single or dual flow cell configurations (see associatedtable). Thus, the time required to sequence 80 bases at each position onthe array within the eight flow cells would be approximately 11T (seeassociated table). In sum, by increasing the number of flow cells (toreflect the number of sequencing steps or at least approximate thenumber of sequencing steps) and reducing the area to be imaged per flowcell, the number of cycles per unit time is increased such that reachinga desired sequence read length can be reduced by almost one half.

Further advantages (discussed in more detail below) are achieved bybeing able to move flow cells in a manner that they need not return totheir prior position. For example, in one embodiment, flow cells areoff-loaded (e.g. for imaging) from the moving support at a firstposition (e.g. the last position in the cycle), and loaded back onto themoving support at a second position (preferably the second positionallows the flow cell to either return to the same cycle, or start a newsequencing cycle, without passing through non-functional stations ordwell stations).

It is not intended that the present invention be limited by the natureof the plate or support (e.g. glass, metal, etc.) for the flow cells orthe nature of the movement (e.g. x/y, up/down or circular) of the plateor support which comprises the plurality of smaller flow cells; however,a circular support (in the manner of a carousel) is preferred andprovides certain advantages described herein.

In one embodiment, a first flow cell is moved to a first station for thefirst step of the sequencing protocol. When the first step is complete,the first flow cell is moved to a second station in order to carry outthe second step of the sequencing protocol. At the same time, a secondflow cell is moved to the first station for the first step of thesequencing protocol. In this embodiment, multiple flow cells are run inparallel at different stages in the protocol. In this sequentialapproach, the first flow cell will complete the sequencing protocolprior to the second flow cell, allowing replacement of the first flowcell with a new flow cell for processing, even though the other flowcells require additional processing. Using “removeable” flow cells inthis manner, a researcher can gain access to a shared machine (e.g. in acore facility) without waiting for the completion of processing for allflow cells. Furthermore, not all flow cells need to be processed inevery run; some “slots” can be “open” for the introduction of a new flowcell (e.g. only two flow cells might be “running” on a carousel whichhas a capacity for eight or more flow cells).

Alternatively, in another embodiment, a plurality of smaller flow cellsare processed simultaneously, not sequentially. This has thedisadvantage that a researcher cannot gain access to the shared machineuntil the processing is complete for all flow cells.

Connections, controls, seals and fluidics at each station are designedwith due concern for the potential for contamination (i.e. possiblecarryover of reagent in a later step from an earlier step in theprotocol). For example, in one embodiment, seal areas between the flowcell and the inlet and outlet to the flow cell at the wash station arelarger than the seal areas at the reagent stations in order to clean thesurface at the seal interface so as to not contaminate the flow cellwith reagents from previous steps.

Each station can have a pump or there can be fewer pumps than stations.Preferably, a single pump can be employed with valving for the differentstations (e.g. commercially available syringe pumps have such valving).Reagent sources are preferably positioned below the flow cells, withtubing coming up to the flow cell through inlet ports (described morefully in connection with the drawings below).

In one embodiment of the system and method, there are 5 chemistry(reagent delivery) stations, 1 imaging station, 1 load/unload stationand 12-14 dwell stations. This system can accommodate from 1 to 20 flowcells and it will run with any number of flow cells up to 20. One couldconfigure the system to have a max number of flow cells of almostanything independent of the number of steps. In one embodiment of thesystem and method, there are 4-7 chemistry stations, 1-2 imagingstation, 1-2 load/unload stations and 12-14 dwell stations. This systemcan accommodate from 1 to 20 flow cells and it will run with any numberof flow cells up to 20. Again, one could configure the system to have amax number of flow cells of almost anything independent of the number ofsteps. Thus, in one embodiment, the number of flow cells exceeds thenumber of steps in any one cycle of a sequencing protocol, e.g. wherethe number of flow cells is twice the number of steps, or three timesthe number of steps, or even four times the number of steps or more.

FIG. 61 shows one embodiment of the system and method with twentyequally spaced locations, numbered 1 to 20, around a circular carouselthat represent positions where the flow cells on the carousel may stop.Only ten flow cells are shown in every other location for clarity;however, as many as twenty fixtures can be on the carousel to locatefrom one to twenty flow cells. These flow cells are sequenced throughthe set of extension, washing, imaging, cleaving, and washing stepswhich cover ten locations around the system. An additional tennon-functional locations are provided to expand the diameter of thecarousel and, therefore, the number of samples (flow cells) that may berun at one time. If the carousel is not fully populated with twenty flowcells, then some or all of the non-functional locations may be skipped,making the system a semi-synchronous one rather than a fully synchronousone. In this embodiment, the imaging station removes the flow cells fromthe carousel for scanning. The station may also replace the flow cell ata different (empty) position on the carousel from where the flow celloriginally resided in order to skip over certain non-functionallocations. Thus, even when the system is populated with fewer than tenflow cells, no extra time is required to allow flow cells to indexthough the non-functional locations.

FIG. 61 shows an embodiment wherein a plurality of locations in thesystem are heated (e.g. to between 37° C. and 70° C., and preferably 55°C.) to facilitate various steps in the sequencing reaction. FIG. 61shows that a subset of the locations have a fluidics station that canflow reagents or washes thorough the flow cells located at thosestations. Some stations are dwell stations where flow cells that werefilled in previous stations are continued to be maintained at theappropriate temperature to allow the reactions to complete shouldreactions times be more than the indexing time.

FIG. 62 shows that additional flow cells beyond five will increase thecycle time. The run times for 1 through 10 flow cells are shown in aMINI system that indexes every 2.5 minutes and 50 cycles (rotations) forevery flow cell. One rotation equals one additional base in read length.It should be noted that up to five flow cells may be run in the systemwith the shortest possible cycle times (about 25 minutes) and anyadditional flow cells above five will slow the overall cycle time of thesystem, since additional indexes and dwells are required. FIG. 62 showsthe run times for a 50-base read with varying numbers of relativelysmall flow cells (e.g. flow cells with areas that can support 20 millionreads). Thus, 50 bases long reads for five flow cells generates 5gigabases of sequence in less than a day and 50-base reads for a fullypopulated run of 20 flow cells will generate 20 Gigabases of sequence in3.5 days.

If the system is run with larger format flow cells (e.g. flow cells thathave 100 million reads), then the throughput for a 50-base run is shownin FIG. 63. In this configuration, the run time is primarily dependenton the time is takes to image a chip and the advantage of breaking upthe chemistry steps is less important in determining the run time.Twenty flow cells of this configuration will generate 100 Gigabases ofsequence in a 50-base run; however it will take about 17 days tocomplete. As a practical matter, a lab that has large projects requiringsuch ultra-high-throughput runs would likely have a system designedspecifically for such runs.

The invention provides methods, compositions, devices, systems and kitsare described including, without limitation, reagents, mixtures, dataprocessing steps, algorithms, computer readable media, and computerprograms, for determining the identity of nucleic acids in nucleotidesequences using, for example, data obtained from sequencing by synthesismethods. The methods of the invention include reducing and/or correctingone or more phenomena that are encountered during nucleotide sequencing,such as using sequencing by synthesis methods. These phenomena include,without limitation, sequence lead, sequence lag, spectral crosstalk,light from neighboring spots, and noise resulting from variations inillumination and/or filter responses.

In one embodiment, the present invention contemplates a set of dataprocessing steps that may be used to analyze images of a hexagonal arrayof spots or beads on a surface. In one embodiment, the steps comprise a)field flattening and background subtraction, b) spot location in thearray, c) image sharpening, d) spot brightness determination, e)neighbor influence elimination, and f) color crosstalk elimination. Eachof these steps is described in more detail below. Of course, in oneembodiment, the present invention contemplates using a subset of thesesteps (in the same order or in a different order) as well as additionalprocessing steps. The result of the analysis may be used to makemeasurements of the output of four different fluorescent dyes for eachspot in the array. The methods described may also be generalized for arectangular or other shaped arrays rather than a hexagonal array.

In one embodiment, the invention provides a method for determining anidentity of a nucleic acid at an interrogation position in a nucleotidesequence from data acquired from one or more channels, comprising a)obtaining a data set for one or more probe intensities at one or morenucleic acid positions in the sequence, wherein each probe correspondsto a nucleic acid, b) determining the ratio contribution to probeintensity at the interrogation position from probe intensities at theinterrogation position and at one or both of i) at least one subsequentnucleic acid positions in the sequence, and ii) at least one precedingnucleic acid positions in the sequence, and c) applying the ratiocontribution to probe intensity to the data set to arrive at an identityfor a nucleic acid at the interrogation position in the nucleotidesequence. In a particular embodiment, the step of determining the ratiocontribution to probe intensity comprises measuring the rate (that is,the fraction of template molecules in an ensemble of identical templatemolecules) at which a lag, such as Gi, occurs at one or more nucleotideposition in the nucleotide sequence, such as at each nucleotide positionin the nucleotide sequence. In another embodiment, the step ofdetermining the ratio contribution to probe intensity comprisesmeasuring the rate (fraction) at which a lead, such as Di, occurs at oneor more nucleotide positions in the nucleotide sequence. In yet anotherembodiment, the method further comprises calling a nucleic acid at theinterrogation position in the nucleotide sequence. In a furtherembodiment, the method comprises repeating steps b) and c) to arrive atan identity for a nucleic acid at more than one interrogation positionin the nucleotide sequence.

While not intending to limit the invention's method to particular steps,in one embodiment, the method further comprises a) applying a sequencelead-lag compensation equation to determine the ratio contribution toprobe intensity from probe at i) the interrogation position, ii) eachposition preceding the interrogation position, and iii) each positionsubsequent to the interrogation position, and b) summing up the ratiocontribution to probe intensity. In an alternative embodiment, the stepof applying of the ratio contribution to probe intensity comprises a)comparing probe intensities from the one or more channels at theinterrogation position, b) selecting the highest probe intensity of thecompared probe intensities, and c) calling a nucleic acid, whichcorresponds to the selected probe, at the interrogation position.

It is not intended to limit the invention to a particular mathematicalformula. Nonetheless, in one embodiment, the method comprises applying asequence lead-lag compensation equation to the ratio contribution toprobe intensity at a plurality of positions in the sequence. In oneparticular embodiment, the sequence lead-lag compensation equation isdetermined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix} = {K_{{Lead}\text{/}{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix}}$

where

I_(M1) is a probe intensity measured at position 1 in the sequence,

I_(M2) is a probe intensity measured at position 2 in the sequence,

I_(MN) is a probe intensity measured at position N in the sequence,

I_(A1) is the actual probe intensity at position 1 in the sequence,

I_(A2) is the actual probe intensity at position 2 in the sequence,

I_(AN) is the actual probe intensity at position N in the sequence,where

$K_{{Lead}\text{/}{Lag}} = \begin{bmatrix}R_{{{Lag}\text{/}{Lead}},1} & R_{{{+ 1}{Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}\text{/}{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},3} & R_{{{- 1}{Lag}},3} & R_{{{Lag}\text{/}{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}\text{/}{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}\text{/}{Lead}},N}\end{bmatrix}$

where

-   -   R_(Lag/Lead,1) is the ratio between reduced probe intensity for        nucleic acid at position 1 to actual probe intensity at the        nucleic acid at position 1,    -   R_(+1Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 2,    -   R_(+2Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 3,    -   R_(+3Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 4,    -   R_(+(N−1)Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position        1+(N−1),    -   R_(−1Lag,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 1,    -   R_(Lag/Lead,2) is the ratio between reduced probe intensity for        nucleic acid at position 2 to actual probe intensity at the        nucleic acid at position 2,    -   R_(+1Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 3,    -   R_(+2Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 4,    -   R_(+(N−2)Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position        2+(N−2),    -   R_(−2Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 1,    -   R_(−1Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 2,    -   R_(Lag/Lead,3) is the ratio between reduced probe intensity for        nucleic acid at position 3 to actual probe intensity at the        nucleic acid at position 3,    -   R_(+1Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 4,    -   R_(+(N−3)Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position        3+(N−3),    -   R_(−3Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 1,    -   R_(−2Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 2,    -   R_(−1Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 3,    -   R_(Lag/Lead,4) is the ratio between reduced probe intensity for        nucleic acid at position 4 to actual probe intensity at the        nucleic acid at position 4,    -   R_(+(N−4)Lead,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position        4+(N−4),    -   R_(−(N−1)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−1),    -   R_(−(N−2)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−2),    -   R_(−(N−3)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−3),    -   R_(−(N−4)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−4), and    -   R_(Lag/Lead,N) is the ratio between reduced probe intensity for        nucleic acid at position N to actual probe intensity at the        nucleic acid at position N.        In a further embodiment, the sequence lead-lag compensation        equation is determined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix} = {{K_{{Lead}\text{/}{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix}}.}$

In a particular embodiment, the nucleic acid comprises a base selectedfrom the group of adenine (A), guanine (G), cytosine (C), thymine (T),and uracil (U), and the probe is detectable using any means such ascolor in the visible spectrum (e.g., fluorescence), radioactivity, andthe like.

While not intending to limit the invention's methods to particularsteps, in one embodiment, the methods further comprise field flatteningof background data for the data set. This may be accomplished by, forexample, obtaining a first data set for a plurality of pixel intensitiesof a first raw image of a probe at a first concentration on a solidsupport, wherein the first raw image is produced using a first spectralfilter for detecting a first probe, b) obtaining a second data set for aplurality of pixel intensities of a second smoothed image of the probeuniformly spread on the solid support or other uniformly radiatingsubstrate, wherein the second smoothed image is produced using a lowpass filter, c) determining a field flattening intensity value for aplurality of pixels of the first raw image, and d) generating a thirdfield flattened image of the probe on the solid support using the fieldflattening intensity of the plurality of pixels, wherein the correlationof intensity of a plurality of pixels to their spatial location on thethird field flattened image is reduced compared to the intensity of aplurality of pixels at a corresponding location on the first raw image.In a particular embodiment, the background intensities are removed fromboth the first and second data sets so that the lowest intensity datapoint is at 0.

Although the field flattening methods are not intended to be limited toany particular equation, in one embodiment, the field flatteningintensity value of a pixel is determined by equation

F _(x,y) =R _(x,y) M _(x0,y0) /M _(x,y)

where

-   -   F_(x,y) is a field flattening intensity value,    -   R_(x,y) is the intensity of a pixel of the plurality of pixels        on the first raw image,    -   M_(x,y) is the intensity of a pixel of the plurality of pixels        on the second smoothed image at a corresponding spatial location        to the pixel on the first raw image, and    -   M_(x0,y0) is the intensity of a reference pixel on the second        smoothed image or is an arbitrary scale factor.        In one embodiment, the scale factor M_(x0,y0) may also include a        factor accounting for different exposure times or lighting        intensities. In another embodiment, such as where a camera        system has a proportional response to changes in exposure times        or lighting conditions, the following equation may be used

M _(x0,y0) =M ₀ E _((second image)) /E _((first image))

where E_((first image)) is the exposure or lighting level used duringmeasurement of the first image, E_((second image)) is the exposure orlighting level used for the second image and M₀ is an arbitraryconstant. In a further embodiment, the method further comprisesrepeating steps a) to d), using a second spectral filter for detecting asecond probe. In an alternative embodiment, the method further comprisesrepeating steps a) to d), using the probe at a second concentration onthe solid support. The solid support is exemplified, but not limited to,a microscope slide and silicon chip.

Also without limiting the invention's methods to particular steps, inone embodiment, the methods further comprise reducing spectral crosstalkin the one or more channels, by a) determining spectral crosstalkfactors for each of the one or more probes in its corresponding channelfrom one or more adjacent channels, b) applying the spectral crosstalkfactors to determine a spectral crosstalk matrix, and c) applying thespectral crosstalk matrix to the data set for the one or more probeintensities. In a particular embodiment, the step of reducing spectralcrosstalk comprises a) determining probe intensity for one or moreprobes from one or more channels, wherein each channel corresponds to aprobe, b) determining the ratios of the probe intensities in the one ormore channels to arrive at signature ratios for the probe intensity inthe channels, c) applying the signature ratios in a matrix equation, andd) inverting the matrix equation to arrive at an inverted matrix. In oneembodiment, the method further comprises e) applying the inverted matrixto data from the one or more channels.

While not intending to limit reducing spectral crosstalk to anyparticular equation, in one embodiment, the step of determining spectralcrosstalk matrix comprises using equation

$\begin{matrix}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}} \\{where} \\{K = \begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{DC} & 1\end{bmatrix}}\end{matrix}$

and where

-   -   M_(A) is the observed intensity in the channel for probe A,    -   M_(B) is the observed intensity in the channel for probe B,    -   M_(C) is the observed intensity in the channel for probe C,    -   M_(D) is the observed intensity in the channel for probe D,    -   A is the actual probe intensity of probe A,    -   B is the actual probe intensity of probe B,    -   C is the actual probe intensity of probe C,    -   D is the actual probe intensity of probe D,    -   R_(AB) is the ratio between (a) the portion of intensity in the        channel for probe A that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(BA) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe A, and (b) the        actual probe intensity of probe A,    -   R_(BC) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe C, and (b) the        actual probe intensity of probe C,    -   R_(CB) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(CD) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe D, and (b) the        actual probe intensity of probe D, and    -   R_(DC) is the ratio between (a) the portion of intensity in a        channel for probe D that is contributed by probe C, and (b) the        actual probe intensity of probe C.        The above equation is solved to determine spectral crosstalk        matrix K⁻¹ and an estimate of the actual intensities of the        probes (A, B, C and D) using equation

$\begin{bmatrix}A \\B \\C \\D\end{bmatrix} = {K^{- 1}\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}$

In an alternative embodiment, the equation is solved to determine and/orestimate for actual probe intensities A, B, C and D.

The invention further provides an algorithm for processing data fornucleic acids in a nucleotide sequence, wherein the data is acquiredfrom one or more channels, the algorithm comprising a) determining theratio contribution to probe intensity in the one or more channels forone or more interrogation positions, from probe intensities at theinterrogation position and at one or both of i) at least one subsequentnucleic acid positions in the sequence, and ii) at least one precedingnucleic acid positions in the sequence, b) processing data from the oneor more channels to correct for sequence lead and sequence lag, and c)reconstructing the data in the one or more channels. In one embodiment,the step of processing data comprises applying the ratio contribution toprobe intensity to determine, for the probe at the one or moreinterrogation positions, a sequence lead-lag compensation equation.Without limiting the invention to any particular equation, in oneembodiment, the sequence lead-lag compensation equation is determined byapplying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix} = {K_{{Lead}\text{/}{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix}}$

where

-   -   I_(M1) is a probe intensity measured at position 1 in the        sequence,    -   I_(M2) is a probe intensity measured at position 2 in the        sequence,    -   I_(MN) is a probe intensity measured at position N in the        sequence,    -   I_(A1) is the actual probe intensity at position 1 in the        sequence,    -   I_(A2) is the actual probe intensity at position 2 in the        sequence,    -   I_(AN) is the actual probe intensity at position N in the        sequence,        In an alternative embodiment, the sequence lead-lag compensation        equation is determined by applying equation

$K_{{Lead}\text{/}{Lag}} = \begin{bmatrix}R_{{{Lag}\text{/}{Lead}},1} & R_{{{+ 1}{Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}\text{/}{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},3} & R_{{{- 1}{Lag}},3} & R_{{{Lag}\text{/}{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}\text{/}{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}\text{/}{Lead}},N}\end{bmatrix}$

where

-   -   R_(Lag/Lead,1) is the ratio between reduced probe intensity for        nucleic acid at position 1 to actual probe intensity at the        nucleic acid at position 1,    -   R_(+1Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 2,    -   R_(+2Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 3,    -   R_(+3Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position 4,    -   R_(+(N−1)Lead,1) is the ratio contribution to probe intensity at        nucleic acid position 1 from probe at nucleic acid position        1+(N−1),    -   R_(−1Lag,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 1,    -   R_(Lag/Lead,2) is the ratio between reduced probe intensity for        nucleic acid at position 2 to actual probe intensity at the        nucleic acid at position 2,    -   R_(+1Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 3,    -   R_(+2Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position 4,    -   R_(+(N−2)Lead,2) is the ratio contribution to probe intensity at        nucleic acid position 2 from probe at nucleic acid position        2+(N−2),    -   R_(−2Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 1,    -   R_(−1Lag,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 2,    -   R_(Lag/Lead,3) is the ratio between reduced probe intensity for        nucleic acid at position 3 to actual probe intensity at the        nucleic acid at position 3,    -   R_(+1Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position 4,    -   R_(+(N−3)Lead,3) is the ratio contribution to probe intensity at        nucleic acid position 3 from probe at nucleic acid position        3+(N−3),    -   R_(−3Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 1,    -   R_(−2Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 2,    -   R_(−1Lag,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position 3,    -   R_(Lag/Lead,4) is the ratio between reduced probe intensity for        nucleic acid at position 4 to actual probe intensity at the        nucleic acid at position 4,    -   R_(+(N−4)Lead,4) is the ratio contribution to probe intensity at        nucleic acid position 4 from probe at nucleic acid position        4+(N−4),    -   R_(−(N−1)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−1),    -   R_(−(N−2)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−2),    -   R_(−(N−3)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−3),    -   R_(−(N−4)Lag,N) is the ratio contribution to probe intensity at        nucleic acid position N from probe at nucleic acid position        N−(N−4), and    -   R_(Lag/Lead,N) is the ratio between reduced probe intensity for        nucleic acid at position N to actual probe intensity at the        nucleic acid at position N.        In another alternative embodiment, the sequence lead-lag        compensation equation is determined by applying equation

$\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix} = {{K_{{Lead}\text{/}{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix}}.}$

While not necessary, it may be desirable to also include fieldflattening of background data in the algorithm and/or reducing spectralcrosstalk between the data comprised in a plurality of channels.Dephasing correction (i.e., correction for lead-lag effects), fieldflattening and spectral crosstalk correction may be carried out in anyorder. Thus, in one embodiment, the field flattening is carried outbefore spectral crosstalk correction. In an alternative embodiment,spectral crosstalk correction is carried out before dephasingcorrection.

The invention also provides a computer readable medium containing acomputer program for performing one or more of the method stepsdisclosed herein.

Also provided by the invention is a computer program product forprocessing data for nucleic acids in a nucleotide sequence to determinean identity of a nucleic acid at an interrogation position in thenucleotide sequence, the computer program product comprising a) computercode that inputs data from one or more channels for one or more probeintensities, wherein each channel corresponds to a probe, and each probecorresponds to a nucleic acid, b) computer code that applies to theinput data a sequence lead-lag compensation equation to correct forsequence lead and sequence lag, c) computer code that compares probeintensities in the one or more channels that have been corrected forsequence lead and sequence lag, d) computer code that determines thehighest probe intensity of the compared probe intensities, and e)computer code that identifies a nucleic acid at the interrogationposition according to the highest probe intensity. Optionally, thecomputer program product may further comprise computer code that appliesfield flattening of background data and/or that reduces spectralcrosstalk between data comprised in the one or more channels.

The invention also provides an apparatus that processes data for nucleicacids in a nucleotide sequence to determine an identity of a nucleicacid at an interrogation position in the nucleotide sequence, theapparatus comprising a) means for inputting data from one or morechannels for one or more probe intensities, wherein each channelcorresponds to a probe, and each probe corresponds to a nucleic acid, b)means for applying to the input data a sequence lead-lag compensationequation to correct for sequence lead and sequence lag, c) means forcomparing probe intensities in the one or more channels that have beencorrected for sequence lead and sequence lag, d) means for determiningthe highest probe intensity of the compared probe intensities, and e)means for identifying a nucleic acid at the interrogation positionaccording to the highest probe intensity. Though not necessary, it maybe desirable to also include means for applying field flattening ofbackground data and/or for reducing spectral crosstalk between datacomprised in the one or more channels.

Additionally provided herein is a system for processing data todetermine an identity of a nucleic acid at an interrogation position inthe nucleotide sequence, the system comprising a) a processor, and b) acomputer readable medium readable by the processor, the computerreadable medium storing a computer program that comprises i) code thatreceives as input a plurality of probe intensities at various positionsin a nucleotide sequence, ii) code that applies to the input data asequence lead-lag compensation equation to correct for sequence lead andsequence lag, and iii) code that identifies a nucleic acid at one ormore interrogation position according to the corrected data. While notnecessary, it may be desirable to additionally include in the computerreadable medium code that applies field flattening of background dataand/or that reduces spectral crosstalk between data comprised in the oneor more channels.

The invention also provides a method for field flattening an image of aprobe on a solid support, comprising a) obtaining a first data set for aplurality of pixel intensities of a first raw image of a probe at afirst concentration on a solid support, wherein the first raw image isproduced using a first spectral filter for detecting a first probe, b)obtaining a second data set for a plurality of pixel intensities of asecond smoothed image of the probe on the solid support, wherein thesecond smoothed image is produced using a low-pass filter, c)determining a field flattening intensity value for a plurality of pixelsof the first raw image, and d) generating a third field flattened imageof the probe on the solid support using the field flattening intensityof the plurality of pixels, wherein the correlation of intensity of aplurality of pixels to their spatial location on the third fieldflattened image is reduced compared to the intensity of a plurality ofpixels at a corresponding location on the first raw image. Withoutintending to limit the invention to any particular equation, in oneembodiment, the field flattening intensity value of a pixel isdetermined by equation

F _(x,y) =R _(x,y) M _(x0,y0) /M _(x,y)

where

-   -   F_(x,y) is a field flattening intensity value,    -   R_(x,y) is the intensity of a pixel of the plurality of pixels        on the first raw image,    -   M_(x,y) is the intensity of a pixel of the plurality of pixels        on the second smoothed image at a corresponding spatial location        to the pixel on the first raw image, and    -   M_(x0,y0) is the intensity of a reference pixel on the second        smoothed image, or is any other scale factor of interest.

In one embodiment, it may be desirable to repeat steps a) to d), using asecond spectral filter for detecting a second probe. Alternatively, orin addition, it may be desirable to repeat steps a) to d), using theprobe at a second concentration on the solid support. In one embodiment,the probe is fluorescent and corresponds to a nucleic acid thatcomprises a base selected from the group of adenine (A), guanine (G),cytosine (C), thymine (T), and uracil (U). The solid support maycomprise a microscope slide, silicon chip, and the like.

The invention also provides a method for reducing spectral crosstalk inone or more channels that deliver data for determining the identity of anucleic acid at an interrogation position in a nucleotide sequence,comprising a) obtaining a data set for one or more probe intensities atone or more nucleic acid positions in the sequence, wherein each probecorresponds to a nucleic acid, b) determining spectral crosstalk factorsfor each of the one or more probes in its corresponding channel from oneor more adjacent channels, c) applying the spectral crosstalk factors todetermine a spectral crosstalk matrix, and d) applying the spectralcrosstalk matrix to the data set to arrive at an identity for a nucleicacid at the interrogation position in the nucleotide sequence. In oneembodiment, the step of determining spectral crosstalk factors comprisesdetermining a ratio between (a) the portion of probe intensity in afirst channel of a first probe that is contributed by a second probe ina second channel adjacent to the first channel, and (b) the actual probeintensity of the second probe in the second channel. In a particularembodiment, the method further comprises determining the ratio between(a) the portion of probe intensity in the first channel of the firstprobe that is contributed by a third probe in a third channel adjacentto the first channel, and (b) the actual probe intensity of the thirdprobe in the third channel. Without limiting the type of equation used,in one embodiment, the step of determining spectral crosstalk matrixcomprises using equation

$\begin{matrix}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}} \\{where} \\{K = \begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{DC} & 1\end{bmatrix}}\end{matrix}$

and where

-   -   M_(A) is the observed probe intensity of probe A,    -   M_(B) is the observed probe intensity of probe B,    -   M_(C) is the observed probe intensity of probe C,    -   M_(D) is the observed probe intensity of probe D,    -   A is the actual probe intensity of probe A,    -   B is the actual probe intensity of probe B,    -   C is the actual probe intensity of probe C,    -   D is the actual probe intensity of probe D,    -   R_(AB) is the ratio between (a) the portion of intensity in the        channel for probe A that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(BA) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe A, and (b) the        actual probe intensity of probe A,    -   R_(BC) is the ratio between (a) the portion of intensity in the        channel for probe B that is contributed by probe C, and (b) the        actual probe intensity of probe C,    -   R_(CB) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe B, and (b) the        actual probe intensity of probe B,    -   R_(CD) is the ratio between (a) the portion of intensity in a        channel for probe C that is contributed by probe D, and (b) the        actual probe intensity of probe D, and    -   R_(DC) is the ratio between (a) the portion of intensity in a        channel for probe D that is contributed by probe C, and (b) the        actual probe intensity of probe C.        In a further embodiment, the equation is solved to determine        spectral crosstalk matrix K⁻¹ and an estimate of the actual        intensity or probes (A, B, C and D) using equation

$\begin{bmatrix}A \\B \\C \\D\end{bmatrix} = {K^{- 1}\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}$

In a particular embodiment, the order of the data correction methodsdescribed herein is 1) field flattening, 2) color crosstalk correctionand 3) dephasing correction. When field flattening precedes colorcrosstalk correction, then the same crosstalk parameters may be used forthe entire image. When color crosstalk correction precedes dephasingcorrection, the dephasing correction will be more accurate as theintensity data from the different channels will more precisely representactual probe intensities.

As noted above, the present invention contemplates reducing some ofthese phenomenon that make accurate base calling difficult. One problemaddressed in one embodiment of the present invention is the problemcreated by using a cleaving agent. In one embodiment, a cleaving agentscavenger is employed to address (e.g. neutralize or otherwise renderinactive) leftover cleaving agent which might prematurely cleave in thenext cycle. Thus, the present invention contemplates in one embodiment amethod of incorporating labeled nucleotides into nucleic acid,comprising: a) providing a plurality of nucleic acid template molecules,a polymerase, a cleaving agent, a cleaving agent scavenger, and aplurality of nucleotide analogues wherein each nucleotide analogue islabeled with a unique label and contains a removable chemical moietycapping the 3′-OH group; b) incorporating a first nucleotide analoguewith said polymerase; c) detecting the label of the incorporatednucleotide analogue; d) removing the chemical moiety of the incorporatednucleotide analogue capping the 3′-OH group with said cleaving agent;and f) incorporating a second nucleotide analogue in the presence ofsaid cleaving agent scavenger. With regard to step f), the scavengercan, by way of example, be put into the solution used to incorporatenucleotides in the next round (indeed, in one embodiment, the presentinvention contemplates compositions comprising 1) the scavenger(s) andone or more labeled or unlabeled nucleotides, 2) the scavenger(s) andpolymerase, 3) the scavenger(s) and one or more nucleotides with orwithout 3′-OH capping groups). Alternatively, the scavenger can be in aseparate solution that is used prior to the incorporation solution (withresidual scavenger present at the time of incorporation). In oneembodiment, the present invention contemplates wash steps after step b)and after step d). In one embodiment, the nucleotide analogues compriseacyclic sugars.

It is not intended that the present invention be limited by the natureof the chemistry of the removable chemical moiety. A variety ofchemistries are contemplated (and described below in more detail). Inone embodiment, said removable chemical moiety comprises a disulfidebond. In another embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether). It is preferred that saidmoiety capping the 3′-OH is not a fluorescent moiety.

It is also not intended that the present invention be limited by thenature of the cleaving agent. In the case of azido-group-containingnucleotides (e.g. 3′-O-azidomethyl ether nucleotides), several types ofcleaving agents can be used. In principle, any reducing agent capable ofconverting the azido group into an amine is suitable for this purpose.The amine undergoes spontaneous conversion to hydroxyl group to enablenext nucleotide incorporation. Examples of cleaving agents include: a)Catalytic hydrogenation over PtO2 or Pd/C; b) Reduction with LiAlH4,HCO₂NH₄-10% Pd/C, NaBH₄/CoCl₂.6H₂O, Zn/NH₄Cl, Fe/NH₄Cl; and c) Reductionwith phosphines; e.g. tri-n-butyl-phosphine, triphenyl phosphine and itssulfonated versions (i.e., tris(3-sulfophenyl)-phosphine, TPPTS), andtri(carboxyethyl)phosphine (TCEP) and its salts. Most preferred cleavingreagents are soluble in water and are highly selective reducing agents.Water soluble phosphines are particularly preferred. In one embodiment,said cleaving agent is a phosphine Tris(2-carboxy-ethyl)phosphine.

It is also not intended that the present invention be limited by thenature of the cleaving agent scavenger. A variety of chemistries arecontemplated (and are described below and in the figures) and more thanone type of chemistry can be used together (e.g. two differentscavengers). In a preferred embodiment, said cleaving agent scavengerdoes not contain a nucleic acid base. In one embodiment, said cleavingagent scavenger comprises a disulfide bond (e.g. cystamine or one of theother disulfide-containing compounds shown in FIG. 37). Cystamine isalso known as 2,2′-Dithiobisethanamine, 2-Aminoethyl disulfide, orDecarboxycystine, and is available commercially from Sigma-Aldrich.Alternatively, the present invention contemplates in one embodiment thatsaid cleaving agent scavenger comprises an azido group (e.g. anazidomethyl group, an azidoethyl ether group, etc.). In a preferredembodiment, said scavenger is 11-Azido-3,6,9-trioxaundecan-1-amine(which is also known as: 1-Amino-11-azido-3,6,9-trioxaundecane,2-{2-[2-(2-Azidoethoxy)ethoxy]ethoxy}ethylamine, orO-(2-Aminoethyl)-O′-(2-azidoethyl)-diethylene glycol, and which isavailable commercially from Sigma-Aldrich).

It is not intended that the present invention be limited by where thefirst and second nucleotides are incorporated. In one embodiment, theyare incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

In some embodiments, two cites of cleavage are contemplated, i.e.cleavage occurs at two locations on the nucleotide analogue. Thus, inone embodiment, the present invention contemplates a method ofincorporating labeled nucleotides into nucleic acid, comprising: a)providing a plurality of nucleic acid template molecules, a polymerase,a cleaving agent, a cleaving agent scavenger, and a plurality ofnucleotide analogues selected from the group consisting of cytosine,thymine, deaza-adenine and deaza-guanine, wherein each nucleotideanalogue comprises a unique label attached through a cleavable linker toa 5-position of cytosine or thymine or to a 7-position of deaza-adenineor deaza-guanine, and wherein each nucleotide analogue contains aremovable chemical moiety capping the 3′-OH group; b) incorporating afirst nucleotide analogue with said polymerase; c) detecting the labelof the incorporated nucleotide analogue; d) removing the chemical moietyof the incorporated nucleotide analogue capping the 3′-OH group andcleaving the cleavable linker with said cleaving agent; and e)incorporating a second nucleotide analogue in the presence of saidcleaving agent scavenger.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemistry of the removable chemical moiety. A variety ofchemistries are contemplated (and described below in more detail) andthe chemistry need not be the same chemistry as used in the cleavablelinker attaching the label. In one embodiment, said removable chemicalmoiety comprises a disulfide bond. In another embodiment, said removablechemical moiety comprises an azido group (e.g. an azidomethyl ether). Itis preferred that said moiety capping the 3′-OH is not a fluorescentmoiety.

Similarly, a variety of chemistries are contemplated for the cleavablelinker attaching the label to the nucleotide analogue (and these aredescribed in more detail below). In one embodiment, said cleavablelinker comprises a disulfide bond. As noted above, the present inventioncontemplates embodiments wherein the chemistries for the cleavage at thetwo sites is the same, as well as embodiments where it is different. Forexample, in one embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether) and said cleavable linker (whichattaches the label) comprises a disulfide bond. In another embodiment,the cleavable linker comprises an azido group and the removable chemicalmoiety comprises a disulfide bond. In another embodiment, the 3′-aminoxy(3′-ONH2) is used as a reversibly terminating moiety and in yet anotherone comprise hydrocarbyldithiomethyl groups as described in the U.S.Pat. No. 7,279,563, hereby incorporated by reference. Yet anotherembodiment comprises 3′-O—CH2-SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group). In general, a variety ofcleavable linkers can be used for this purpose and are described in theliterature. The linkers may be cleavable chemically, cleavable by lightor by other physical means. WO99/57321 describes the use of nucleotidescomprising fluorophores linked to the nucleotide by chemically orphotochemically cleavable linker moieties (Olejnik et al., Proc. Natl.Acad. Sci., 92:7590-7594 (1995). For example, the linkers could containdisulfide bonds (Mitra, R. D., Shendure, J., Olejnik, J.,Edyta-Krzymanska-Olejnik, and Church, G. M. (2003) Anal Biochem.320(1):55-65) cleavable under reducing conditions (or contain cis-diollinkages cleavable by periodate (WO/53812). The linkers could also becleavable by enzymes (WO 01/92284). Other types of acid or base,oxidative or reducing agent cleavable linkers are also contemplated aswill be recognized by the skilled in the art. Examples of such linkersare described in the literature (Greene et al., Protective Groups InOrganic Synthesis, Second Edition, John Wiley & Sons, (1991).

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers is contemplated (discussedabove). In a preferred embodiment, said cleaving agent scavenger doesnot contain a nucleic acid base.

In one embodiment, the present invention contemplates incorporatingnucleotides having only one location for cleavage (e.g. the cleavablelinker attaching the label). Thus, in one embodiment, the presentinvention contemplates a method of incorporating labeled nucleotidesinto nucleic acid, comprising: a) providing a plurality of nucleic acidtemplate molecules, a polymerase, a cleaving agent, a cleaving agentscavenger, and a plurality of nucleotide analogues wherein eachnucleotide analogue is labeled with a unique label, said label attachedby a cleavable linker; b) incorporating a first nucleotide analogue withsaid polymerase; c) detecting the label of the incorporated nucleotideanalogue; d) removing the label of the incorporated nucleotide analogueby cleaving the cleavable linker with said cleaving agent; and e)incorporating a second nucleotide analogue in the presence of saidcleaving agent scavenger.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, a variety of chemistries is contemplated for the cleavable linker(e.g. wherein said cleavable linker comprises a disulfide bond, azidogroup, or some other chemical group). However, in a preferredembodiment, the chemistry of the cleavable linker dictates the chemistryof the scavenger (e.g. wherein said cleaving agent scavenger comprises adisulfide bond, it is preferred that the scavenger also comprise adisulfide bond, such as where said scavenger is cystamine or othersimilar compound).

In one embodiment, the present invention contemplates carrying outnucleotide incorporation in a device, including automated devices.Solutions comprising various combinations of biomolecules arecontemplated; such solutions can be, in one embodiment, convenientlystored in reservoirs which are in fluid communication with a reactionchamber (e.g. flow cells, microchannels, etc.). A series of steps can becarried out to introduce these solutions (and the reagents they contain)into the reaction chamber (e.g. by valving) to carry out thereaction(s). Thus, in one embodiment, the present invention contemplatesa method of incorporating labeled nucleotides into nucleic acid,comprising: a) providing i) a reaction chamber (e.g. a flow cell)comprising plurality of nucleic acid template molecules bound to a solidsupport, ii) a first solution comprising polymerase and a plurality ofnucleotide analogues wherein each nucleotide analogue is labeled with aunique label and contains a removable chemical moiety capping the 3′-OHgroup, iii) a second solution comprising a cleaving agent, and iv) acleaving agent scavenger; b) introducing said first solution into saidreaction chamber under conditions wherein a first nucleotide analogue isincorporated by said polymerase; c) detecting the label of theincorporated nucleotide analogue; d) introducing said second solutioninto said reaction change under conditions such that the chemical moietyof the incorporated nucleotide analogue capping the 3′-OH group isremoved by said cleaving agent; and e) introducing said cleaving agentscavenger into said reaction chamber.

It is not intended that the present invention be limited by the way inwhich the cleaving agent scavenger is stored or introduced into thereaction chamber. In one embodiment, said cleaving agent scavenger is ina third solution and said scavenger is introduced into said reactionchamber in step e) by introducing said third solution. In anotherembodiment, the above-indicated method further comprises the step f)re-introducing said first solution into said reaction chamber underconditions such that a second nucleotide analogue is incorporated bysaid polymerase (and this first solution may contain the scavenger ifdesired). In another embodiment, separate steps [i.e. step e) and stepf)] are not required; rather, a single step is contemplated wherein saidcleaving agent scavenger is in said first solution and said introducingof step e) comprises introducing said first solution comprising saidscavenger (in this embodiment, a second nucleotide analogue isincorporated in the presence of said cleaving agent scavenger). In someembodiments, additional wash steps are employed to remove reagentsbetween steps [e.g. wash steps after step b), and step d)], although theusefulness of the scavenger has been discovered empirically, sinceresidual cleaving agent is difficult to remove with a practical numberof washes (discussed more below).

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue. In one embodiment, said removable chemical moiety comprises adisulfide bond. In one embodiment, said removable chemical moietycomprises an azido group (e.g. an azidomethyl ether). It is preferredthat said moiety capping the 3′-OH is not a fluorescent moiety. Wherethe protective group residing on the 3′-OH groups of the nucleotides isa 3′-O-azidomethyl group, this group can be removed using mild reducingagents such as tri(carboethoxy)phosphine (TCEP). However, the presentinvention contemplates other protective groups. In another embodiment,the protective groups contemplated by the present invention are 3′-O—NH₂groups. These groups can be removed using in situ generated nitrous acid(such as from sodium nitrite). Similarly to the 3′-O-aminoxy group, the3′-O-methylaminoxy (3′-O—CH₂—NH₂) group can be removed under mildoxidative conditions, for example using in situ generated nitrous acid(such as from sodium nitrite). 3′-O-allyl protective groups can beremoved using variety of reducing agents, including transition metalcomplexes (Pd, Rh). In yet another embodiment, the present inventioncontemplates that the 3′-capping group is the 3′-O—CH₂—SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group). These protective groupscan be removed using variety of reducing agents and cleaved into thecorresponding 3′-OH derivative, for example by usingtris(carboxyethyl)phosphine (TCEP). In yet another embodiment, the3′-capping group is enzymatically cleavable.

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers is contemplated (discussedabove). In a preferred embodiment, said cleaving agent scavenger doesnot contain a nucleic acid base.

In some embodiments, the reaction in the device is directed at cleavageat two locations on the nucleotide analogue(s). Thus, in one embodiment,the present invention contemplates a method of incorporating labelednucleotides into nucleic acid, comprising: a) providing i) a reactionchamber comprising plurality of nucleic acid template molecules bound toa solid support, ii) a first solution comprising polymerase and aplurality of nucleotide analogues selected from the group consisting ofcytosine, thymine, deaza-adenine and deaza-guanine, wherein eachnucleotide analogue comprises a unique label attached through acleavable linker to a 5-position of cytosine or thymine or to a7-position of deaza-adenine or deaza-guanine, and wherein eachnucleotide analogue is labeled with a unique label and contains aremovable chemical moiety capping the 3′-OH group, iii) a secondsolution comprising a cleaving agent (or a second and third solutionincorporating different cleaving solutions), and iv) a cleaving agentscavenger; b) introducing said first solution into said reaction chamberunder conditions wherein a first nucleotide analogue is incorporated bysaid polymerase; c) detecting the label of the incorporated nucleotideanalogue; d) introducing said second solution (and then possibly thethird solution) into said reaction change under conditions such that thechemical moiety of the incorporated nucleotide analogue capping the3′-OH group is removed and said cleavable linker is cleaved by saidcleaving agent; and e) introducing said cleaving agent scavenger intosaid reaction chamber (e.g. flow cell or the like).

Again, it is not intended that the present invention be limited by theway in which the cleaving agent scavenger is stored or introduced intothe reaction chamber. In one embodiment, said cleaving agent scavengeris in a third solution and said scavenger is introduced into saidreaction chamber in step e) by introducing said third solution. Inanother embodiment, the above-indicated method further comprises thestep f) re-introducing said first solution into said reaction chamberunder conditions such that a second nucleotide analogue is incorporatedby said polymerase (and this first solution may contain the scavenger ifdesired). In another embodiment, separate steps [i.e. step e) and stepf)] are not required; rather, a single step is contemplated wherein saidcleaving agent scavenger is in said first solution and said introducingof step e) comprises introducing said first solution comprising saidscavenger (in this embodiment, a second nucleotide analogue isincorporated in the presence of said cleaving agent scavenger). In someembodiments, additional wash steps are employed to remove reagentsbetween steps [e.g. wash steps after step b), and step d)].

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue. In one embodiment, said removable chemical moiety comprises adisulfide bond. In one embodiment, said removable chemical moietycomprises an azido group (e.g. an azidomethyl ether). It is preferredthat said moiety capping the 3-OH is not a fluorescent moiety. Where theprotective group residing on the 3′-OH groups of the nucleotides is a3′-O-azidomethyl group, this group can be removed using mild reducingagents such as tri(carboethoxy)phosphine (TCEP). However, the presentinvention contemplates other protective groups. In another embodiment,the protective groups contemplated by the present invention are 3′-O—NH₂groups. These groups can be removed using in situ generated nitrous acid(such as from sodium nitrite). Similarly to the 3′-O-aminoxy group, the3′-O-methylaminoxy (3′-O—CH₂—NH₂) group can be removed under mildoxidative conditions, for example using in situ generated nitrous acid(such as from sodium nitrite). 3′-O-allyl protective groups can beremoved using variety of reducing agents, including transition metalcomplexes (Pd, Rh). In yet another embodiment, the present inventioncontemplates that the 3′-capping group is the 3′-O—CH₂—SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group). These protective groupscan be removed using variety of reducing agents and cleaved into thecorresponding 3′-OH derivative, for example by usingtris(carboxyethyl)phosphine (TCEP). In yet another embodiment, the3′-capping group is enzymatically cleavable.

Again, the chemistry of the cleavable linker (which attaches the label)may be the same or different vis-à-vis the removable chemical cappingmoiety. Thus, in one embodiment, the linker and the capping groupcomprise a disulfide bond. Yet, in another embodiment, said removablechemical moiety comprises an azido group and said cleavable linkercomprises a disulfide bond (or the reverse, i.e. the capping groupcomprises a disulfide bond and the cleavable linker comprises an azidogroup).

Again, it is also not intended that the present invention be limited bythe nature of the cleaving agent. However, in one embodiment, saidcleaving agent is a phosphine (e.g. Tris(2-carboxy-ethyl)phosphine).Again, a variety of cleaving agent scavengers is contemplated (discussedabove). In a preferred embodiment, said cleaving agent scavenger doesnot contain a nucleic acid base.

In some embodiments, the present invention contemplates a reaction inthe device wherein only a single site of cleavage on the nucleotideanalogue is targeted (e.g. a cleavable linker attaching the label).Thus, in one embodiment, the present invention contemplates a method ofincorporating labeled nucleotides into nucleic acid, comprising: a)providing i) a reaction chamber comprising plurality of nucleic acidtemplate molecules bound to a solid support, ii) a first solutioncomprising polymerase and a plurality of nucleotide analogues whereineach nucleotide analogue is labeled with a unique label, said labelattached via a cleavable linker, iii) a second solution comprising acleaving agent, and iv) a cleaving agent scavenger; b) introducing saidfirst solution into said reaction chamber under conditions wherein afirst nucleotide analogue is incorporated by said polymerase; c)detecting the label of the incorporated nucleotide analogue; d)introducing said second solution into said reaction change underconditions such that the label of the incorporated nucleotide analogueis removed by cleaving said cleavable linker with said cleaving agent;and e) introducing said cleaving agent scavenger into said reactionchamber.

Again, it is not intended that the present invention be limited by wherethe first and second nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

A variety of chemistries for the cleavable linker is contemplated. Inone embodiment, said cleavable linker comprises a disulfide bond.

In one embodiment, the chemistry used in the cleavable linker controlsthe chemistry of the scavenger. For example, in one embodiment, wherethe linker comprises a disulfide bond, said cleaving agent scavengercomprises a disulfide bond. In one embodiment, where the linkercomprises an azido group, said cleaving agent scavenger comprises anazido group. In a preferred embodiment, said cleaving agent scavengerdoes not contain a nucleic acid base.

The present invention contemplates methods, kits, devices, systems andcompositions. In one embodiment, the present invention contemplates acomposition comprising cleaving agent scavenger and one or morenucleotide analogues (unlabeled or labeled as herein described). In oneembodiment, said composition further comprises polymerase. In oneembodiment, the present invention contemplates a composition comprisingcleaving agent scavenger and polymerase, and (optionally) one or morenucleotide analogues (unlabeled or labeled as herein described).

In one embodiment, the present invention contemplates a reaction chamber(e.g. a flow cell, flow channels, etc.) comprising a solution, saidsolution comprising cleaving agent scavenger and one or more nucleotideanalogues (labeled or unlabeled as herein described). In one embodiment,said solution further comprises polymerase. In one embodiment, saidsolution comprises cleaving agent scavenger and polymerase, and(optionally) one or more nucleotide analogues (unlabeled or labeled asherein described).

In one embodiment, the present invention contemplates kits, said kitscomprising a solution comprising cleaving agent scavenger and one ormore nucleotide analogues (labeled or unlabeled as herein described) and(optionally) polymerase. Alternatively, said kits comprise a solutioncomprising cleaving agent scavenger and polymerase, and (optionally) oneor more nucleotide analogues (unlabeled or labeled as herein described).Preferrably, such kits also provide instructions for carrying outincorporation reactions, as well as wash buffers and the like.

In one embodiment, the present invention contemplates a systemcomprising reservoirs in fluid communication with a reaction chamber, atleast one of said reservoirs comprising a solution comprising cleavingagent scavenger and one or more nucleotide analogues (labeled orunlabeled as herein described) and (optionally) polymerase.Alternatively, at least one of said reservoirs comprises a solutioncomprising cleaving agent scavenger and polymerase, and (optionally) oneor more nucleotide analogues (unlabeled or labeled as herein described).Preferrably, such solutions can be introduced by automated means (e.g.valving).

As described herein, the present invention contemplates embodimentswherein nucleotides used in extension reactions contain linkers, spacersand chemical groups. The presence of these spacers and groups may affectthe ability of the sequencing polymerases to incorporate the subsequentnucleotide. The present invention contemplates a number of ways tominimize or eliminate this undesirable effect, including but not limitedto: a) reducing the amount of labeled nucleotides incorporated in thetemplate; b) reducing the size of the spacer arm or eliminate itcompletely by carefully designing nucleotide analogs; and c) change thereactivity of the spacer arm groups or their charge by performing achemical “capping” step, where specific reagent is added to react onlywith groups on the spacer arm.

Reducing the amount of labeled nucleotides that are incorporated can beaccomplished by reducing the concentration of labeled nucleotides in theextension solution, and/or by mixing labeled nucleotides (reversibleterminators) with non-labeled reversibly terminating nucleotides (e.g.where the non-labeled nucleotides are employed in ratios between 1:1 and1000:1 relative to the labeled nucleotides, but more preferably inratios between 10:1 and 100:1). In contrast to labeled nucleotides,non-labeled reversible terminator nucleotides after cleavage convert tonative nucleotide (and therefore do not present problems forpolymerases). Thus, in one embodiment, the present inventioncontemplates a composition comprising i) a first plurality of nucleotideanalogues wherein each nucleotide analogue is labeled with a uniquelabel and contains a removable chemical moiety capping the 3′-OH group;and ii) a second plurality of nucleotide analogues wherein eachnucleotide analogue is unlabeled and contains a removable chemicalmoiety capping the 3′-OH group. In one embodiment, the compositionfurther comprises polymerase. In a preferred embodiment, said nucleotideanalogues are in solution. In one embodiment, the second plurality ofnucleotide analogues is present in said solution at a high concentrationthan said first plurality of nucleotide analogues. In one embodiment,said second plurality of nucleotide analogues is present at aconcentration between 1 uM and 100 uM. In one embodiment, said firstplurality of nucleotide analogues is present at a concentration between1 nM and 1 uM.

It is not intended that the composition be limited by the number ornature of nucleotide analogues in said composition. However, in apreferred embodiment, said first plurality of nucleotide analoguescomprises four different nucleotide analogues (for example, in oneembodiment, the four nucleotides are either (i) aA, aC, aG, and aT, or(ii) aA, aC, aG, and aU). In a preferred embodiment, said secondplurality of (unlabeled) nucleotide analogues comprises four differentnucleotide analogues (for example, either (i) aA, aC, aG, and aT, or(ii) aA, aC, aG, and aU).

It is also not intended that the composition be limited by the nature ofthe label. However, in one embodiment, each of said four differentnucleotide analogues comprises a unique (preferably cleavable) label,said label selected from the group consisting of BODIPY, Rhodamine,Carboxyrhodamine, and Cyanine (see FIG. 36, which shows these labels inthe context of a cleavable disulfide bond).

It is also not intended that the composition be limited by the chemistryof the removable chemical moiety, which may, by way of example, comprisea disulfide bond or an azido group (e.g. an azidomethyl ether). Thechemistry may be the same or different vis-à-vis the cleavable linker.For example, said removable chemical moiety comprises an azido group andsaid cleavable linker comprises a disulfide bond.

In one embodiment, the present invention contemplates a compositioncomprising i) a first plurality of nucleotide analogues comprising fourdifferent (for example, in one embodiment, the four nucleotides areeither (i) aA, aC, aG, and aT, or (ii) aA, aC, aG, and aU) nucleotideanalogues, wherein each different nucleotide analogue is labeled with aunique (preferably cleavable) label and contains a removable chemicalmoiety capping the 3′-OH group; and ii) a second plurality of nucleotideanalogues comprising four different (for example, in one embodiment, thefour nucleotides are either (i) aA, aC, aG, and aT, or (ii) aA, aC, aG,and aU) nucleotide analogues, wherein each nucleotide analogue isunlabeled and contains a removable chemical moiety capping the 3′-OHgroup. Again, this composition may further comprise a polymerase and itis preferred that the reagents (e.g. said nucleotide analogues andoptionally said polymerase) are in solution.

It is not intended that the composition be limited by the particularlinkages. However, in a preferred embodiment, the nucleotide analoguesselected from the group consisting of cytosine, thymine, deaza-adenineand deaza-guanine and each comprising a unique (preferably) labelattached through a cleavable linker to a 5-position of cytosine orthymine or to a 7-position of deaza-adenine or deaza-guanine.

In one embodiment, the second plurality of nucleotide analogues ispresent in said solution at a high concentration than said firstplurality of nucleotide analogues. In one embodiment, said secondplurality of nucleotide analogues is present at a concentration between1 uM and 100 uM. In one embodiment, said first plurality of nucleotideanalogues is present at a concentration between 1 nM and 1 uM.

In one embodiment, the present invention contemplates kits, said kitscomprising a mixture of labeled and unlabeled nucleotide analogues(preferably both containing groups capping the 3′-OH—such as an azidogroup) and (optionally) polymerase. Where the protective group residingon the 3′-OH groups of the nucleotides is a 3′-O-azidomethyl group, thisgroup can be removed using mild reducing agents such astri(carboethoxy)phosphine (TCEP). However, the present inventioncontemplates other protective groups. In another embodiment, theprotective groups contemplated by the present invention are 3′-O—NH₂groups. These groups can be removed using in situ generated nitrous acid(such as from sodium nitrite). Similarly to the 3′-O-aminoxy group, the3′O-methylaminoxy (3′-O—CH₂—NH₂) group can be removed under mildoxidative conditions, for example using in situ generated nitrous acid(such as from sodium nitrite). 3′-O-allyl protective groups can beremoved using variety of reducing agents, including transition metalcomplexes (Pd, Rh). In yet another embodiment, the present inventioncontemplates that the 3′-capping group is the 3′-O—CH₂—SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group). These protective groupscan be removed using variety of reducing agents and cleaved into thecorresponding 3′-OH derivative, for example by usingtris(carboxyethyl)phosphine (TCEP). In yet another embodiment, the3′-capping group is enzymatically cleavable.

In one embodiment, the present invention contemplates a mixture of 4labeled and 4 unlabeled nucleotide analogues as herein described) and(optionally) polymerase. The mixture can be provided dry or in solutionin the kit (along with appropriate instructions for extensionreactions). Preferably, the unlabeled nucleotide analogues are presentin the mixture in a greater amount than the labeled nucleotideanalogues.

The above-indicated solutions provide advantages in incorporationreactions. Thus, in one embodiment, the present invention contemplates amethod of incorporating labeled nucleotides into nucleic acid,comprising: a) providing i) a reaction chamber comprising plurality ofnucleic acid template molecules bound to a solid support, ii) a solutioncomprising a first plurality of nucleotide analogues wherein eachnucleotide analogue is labeled with a unique (preferably cleavable)label and contains a removable chemical moiety capping the 3′-OH group,and a second plurality of nucleotide analogues wherein each nucleotideanalogue is unlabeled and contains a removable chemical moiety cappingthe 3′-OH group; and iii) polymerase; b) introducing said solution intosaid reaction chamber under conditions wherein a nucleotide analogue ofsaid first plurality of nucleotide analogues is incorporated by saidpolymerase (e.g. the polymerase can be added separately or together withother reagents; regardless, it is preferred that said polymerase is insaid solution prior to step b); and c) detecting the label of theincorporated nucleotide analogue. The method may comprise additionalsteps (cleavage of the capping group, washing, etc.) and may repeatsteps (e.g. in order to incorporate subsequent, e.g. a second, third,fourth, etc., nucleotide analogues).

It is not intended that the present invention be limited by where thefirst (or subsequent) nucleotides are incorporated. In one embodiment,they are incorporated into a primer [e.g. prior to step b), the presentinvention contemplates in one embodiment hybridizing a primer to saidplurality of nucleic acid template molecules, such that said firstnucleotide analogue is incorporated into said primer at step b)]. Inanother embodiment, they are incorporated into the template molecules[e.g. said nucleic acid template molecules comprise a self-priminghairpin, such that said first nucleotide analogue is incorporated intosaid template molecules at step b)].

In one embodiment, the second plurality of nucleotide analogues ispresent in said solution at a high concentration than said firstplurality of nucleotide analogues. In one embodiment, said secondplurality of nucleotide analogues is present at a concentration between1 uM and 100 uM. In one embodiment, said first plurality of nucleotideanalogues is present at a concentration between 1 nM and 1 uM.

In a preferred embodiment, said first plurality of nucleotide analoguescomprises four different nucleotide analogues and said second pluralityof nucleotide analogues comprises four different nucleotide analogues.In one embodiment, each of said four different nucleotide analogues ofsaid first plurality of labeled analogues comprises a unique label, saidlabel selected from the group consisting of Alexa, BODIPY, Rhodamine,Carboxyrhodamine, and Cyanine. It is not intended that the presentinvention be limited by the nature of the label or fluorophore used andmany other combinations are contemplated.

Again, it is not intended that the present invention be limited by thenature of the chemical moiety capping the 3′-OH on the nucleotideanalogue or the (preferably cleavable) linker attaching the label. Inone embodiment, said removable chemical moiety comprises a disulfidebond. In one embodiment, said removable chemical moiety comprises anazido group (e.g. an azidomethyl ether). In one embodiment, saidremovable chemical moiety comprises an azido group and said cleavablelinker comprises a disulfide bond. In another embodiment, thesechemistries are reversed. Again, it is preferred that said moietycapping the 3′-OH is not a fluorescent moiety. Where the protectivegroup residing on the 3′-OH groups of the nucleotides is a3′-O-azidomethyl group, this group can be removed using mild reducingagents such as tri(carboethoxy)phosphine (TCEP). However, the presentinvention contemplates other protective groups. In another embodiment,the protective groups contemplated by the present invention are 3′-O—NH₂groups. These groups can be removed using in situ generated nitrous acid(such as from sodium nitrite). Similarly to the 3′-O-aminoxy group, the3′-O-methylaminoxy (3′-O—CH₂—NH₂) group can be removed under mildoxidative conditions, for example using in situ generated nitrous acid(such as from sodium nitrite). 3′-O-allyl protective groups can beremoved using variety of reducing agents, including transition metalcomplexes (Pd, Rh). In yet another embodiment, the present inventioncontemplates that the 3′-capping group is the 3′-O—CH₂—SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group). These protective groupscan be removed using variety of reducing agents and cleaved into thecorresponding 3′-OH derivative, for example by usingtris(carboxyethyl)phosphine (TCEP). In yet another embodiment, the3′-capping group is enzymatically cleavable.

Increasing the number of bases that can be sequenced, i.e. increasingread lengths is desirable. However, as one proceeds to larger and largerread lengths, one often encounters a reduction in signal. In oneembodiment, the present invention contemplates reducing extension times(e.g. extension times of 5-15 minutes are reduced to 1-2 minutes, orless) in order to maintain signal strength at longer read lengths(greater than 20 bases, more preferably greater than 30 bases, etc.).This reduction in extension times can be combined with other methodsherein described (e.g. the use of mixtures of labeled and unlabelednucleotides) to improve performance and increase the retention insignal. Signal retention is defined as the ratio of signals at the endof the run to the signals at the beginning of the run.

Another approach to increasing read lengths involves the use of one ormore oxygen scavengers. In one embodiment, the oxygen scavenger isutilized in an imaging buffer, i.e. a buffer used before, during orafter, and more preferably just prior (with no additional steps inbetween), during, or just after imaging (with no additional steps inbetween). While not intended to limit the invention to any particularmechanism, it is believed that the imaging process generates singletstate oxygen species, e.g. from the exposure of the dyes or labels tothe light, and such species react rapidly with available exposedreactive groups in fluorescent dyes (rendering them non-fluorescent) andwith biomolecules including nucleic acids (causing damage). See Sies,H., and C. F. Menck, “Singlet oxygen induced DNA damage,” Mutat. Res.275:367-375 (1992). Using an oxygen scavenger during the imaging processis believed to reduce this damage. Dyes species in their triplet excitedstate are non-fluorescent, and therefore non-desirable. Furthermore, thetriplet excited state can convert dye molecules into permanentnon-fluorescent state. Therefore, triplet state scavengers can also beintroduced into the sequencing system imaging buffer. Oxygen scavengersinclude compounds that are oxidizable and react with dissolved oxygen.Preferred oxygen scavengers and triplet state quenchers include, but arenot limited to: ascorbic acid and sodium ascorbate, betamercaptoethanol, n-propyl gallate, p-phenylenediamine,1,4-diazabicyclo[2.2.2]octane (DABCO),6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (TROLOX),polyphenol antioxidants, polyvinyl alcohol, butylated hydroxy anisole(BHA), butylated hydroxy toluene (BHT), mercaptoethylamine (MEA),cyclo-octatetraene (COT). See Ono et. Al., J. Histochem. Cytochem., Vol.49, 305-312 (2001); see also Longin, A., C. Souchier, M. Ffrench, and P.A. Bryon, “Comparison of anti-fading agents used in fluorescencemicroscopy: image analysis and laser confocal microscopy study,” J.Histochem. Cytochem. 41:1833-1840 (1993). Another approach to reducingoxygen availability is to use an enzyme-based, oxygen scavenging system.Preferred enzymatic oxygen scavenging systems include, but are notlimited to, the glucose oxidase and catalase system. See M. Landry, P.McCall, Z. Qi, Y. Chemla, Biophys. J., Vol. 97, Issue 8, Pages2128-2136. Alternatively, a protocatechuicacid/protocatechuate-3,4-dioxygenase can be used. See Aitken et. Al.,Biophys J. March 1; 94(5): 1826-1835 (2008). Other suitable enzymaticoxygen scavenging systems are also contemplated alone or in combinationwith quenchers. Finally, a combination of triplet state quenchers andoxygen scavengers can be used (Rasnik et. Al., Nat. Methods. 2006November; 3(11):891-3). In yet another embodiment, one uses anenvironment which is oxygen depleted by using an imaging buffer (whichhas been deoxygenated by purging with helium, argon or nitrogen orthrough exposure to vacuum) under a neutral gas blanket.

Yet another approach to increasing read lengths comprises balancing theamount of nucleotide analogue with the incorporation rate. For example,where a particular nucleotide analogue is incorporated quickly by thepolymerase, the concentration of that particular nucleotide in themixture of nucleotides can be reduced. Similarly, where a particularnucleotide analogue is incorporated slowly, the concentration of thatparticular nucleotide in the mixture can be increased. Importantly,under balanced nucleotide concentration conditions, the misincorporationrate is reduced. After a misincorporation event, the nucleic acid strandis no longer a substrate for the DNA polymerase due to the mismatch andthis contributes to the decline of signal. The balancing can be done byfirst testing solution kinetics of each analog separately. This is bestdone by pre-forming a nucleic acid duplex in solution wherein the primeris labeled with a fluorescent dye on its 5′-end. After incorporation thereaction mixture is then subject to separation using denaturingcapillary electrophoresis with laser-induced fluorescence. Then, kineticparameters of the reaction are calculated and incorporation rates arecompared for the same polymerase and same analog family. The rates arethen normalized to the rate of one of the analogs, the concentrationsadjusted based on the observed rates and then the kinetic studies (timecourse) are repeated and the concentrations refined and matched suchthat the extension reactions for all analogs are complete (100%incorporation) within the same time.

Yet another approach to increasing read lengths comprises purifying thenucleotide analogues to a higher degree of purity (e.g. than typicallyobtained from a commercial supplier) to reduce the amount ofnon-terminating nucleotides present. The presence of non-terminatingnucleotides leads to fast signal dephasing and decline, and thereforemay limit achievable read lengths in SBS. In one embodiment, thenucleotides are purified by orthogonal dual HPLC purification accordingto the following protocol. First, the nucleotides are purified usingion-exchange separation (for example Phenomenex Optisil SAX, anionexchange column) using a salt gradient. In one example separationconditions are as follows: a gradient of 0-50% of B in A over 45minutes, where A=10 mM TEAB, 10% acetonitrile, B=0.85M TEAB, 20%acetonitrile. After this purification, a second purification isperformed using reverse phase column and using a gradient of 0-50%acetonitrile in 50 mM TEAB, pH=8.0 on a reverse phase (e.g., NovaPakC18, reverse phase) column. A variety of other ion exchange columns andreverse phase columns and separation conditions are suitable to performthese purifications. In addition, other separation modes could also beused, such as hydrophobic interaction or size exclusion and combinationmodes. Finally, an enzymatic scavenging system can be used to eliminateany non-terminating nucleotide present after the chromatographicpurification. For example, a reversibly terminating nucleotidepreparation can be pretreated with an extendable DNA duplex and apolymerase that will efficiently utilize non-terminating (native)nucleotides (such as Klenow fragment, exo-) but is not capable ofincorporating reversibly terminating nucleotides. After the treatmentthe nucleotide preparation would be essentially devoid of any nativenucleotides. The scavenging duplex could be designed in such a way thatit would not interfere with the sequencing reaction. An example ofuseful embodiment like this comprises mesophilic scavenging polymerase,low melting temperature scavenging duplex and thermophilic sequencingpolymerase. If sequencing is performed at 55 deg C. for example, thenthe scavenging duplex with low melting point and mesophilic polymerasewould be inactivated. The polymerase can also be immobilized on acolumn, such that the removal of native nucleotides is performed asaffinity chromatography and the nucleotide preps are devoid of thescavenger polymerase.

Yet another approach to increasing read length (e.g. by reducingmisincorporation) is by optimizing the reaction conditions. As notedabove, after a misincorporation event, the nucleic acid strand is nolonger a substrate for the DNA polymerase due to the mismatch and thiscontributes to the decline of signal and reduced read length. Theoptimization of reaction conditions may include, but are not limited to:nucleotide and polymerase concentrations, reaction temperature, pH,monovalent and divalent cation concentrations, ionic strength, cofactorconcentrations, detergents and additives. This approach may also usevarious polymerase mutants and fusion constructs to improve fidelity.

The above approaches can be used separately. However, larger readlengths are achieved when such approaches are combined. Thus, in oneembodiment, the present invention contemplates a) HPLC and/orenzymatically purifying the nucleotide analogues, b) balancing theconcentration of the nucleotide analogues with their incorporation rate,c) utilizing an oxygen scavenger (chemical or enzymatic) and tripletquenching system, e.g. in an imaging buffer used just before, duringand/or after imaging, and d) optimizing reaction conditions. Such acombined approach can improve the signal and thereby improve readlengths significantly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows one embodiment of the imaging system of thepresent invention, said embodiment comprising a) a circular array ofLEDs (101) configured such that the emitted light converges on a regionor platform (e.g. a position for a sample, flow cell, etc.) so as toexcite fluorescence of fluorescent material, b) a lens assembly (102)positioned above the region so as to capture at least a portion of saidfluorescence, c) a filter wheel (103) comprising bandpass filters, andd) light collection means (in this case a cooled CCD camera) (104),wherein said filter wheel is positioned between the region where thelight converges and the light collection means.

FIG. 2 schematically shows one embodiment of a flow cell (200). FIG. 2Ashows a three dimensional translucent view of a flow cell (200),comprising fluid tubing connections, cartridge heaters, and O-ring seal.FIG. 2B is a two dimensional drawing of a side view of a flow cell(200), showing an array or slide (201) with spaced spots on the surface(representing positions for biomolecules and/or anchoring molecules),said array positioned (e.g. inverted) in a fluid channel (202) such thatsolutions of buffers and/or reagents can be introduced over the surfaceunder conditions whereby reactions and/or washing can be achieved. Thearrows show one particular direction of fluid flow, with entrance (204)and exit ports (205), as well as one particular method of sealing(O-ring seal 203). FIG. 2C is a drawing depicting beads (206) in thewells (207) of the slide (or chip 208), which preferably comprisesnucleic acid to be sequenced (not shown), said slide positioned in aflow cell for contact with reagents in the solution traveling throughthe flow cell. The single dark arrow shows reagent flow in the flowcell. The many light arrow heads represent detection (e.g. lightimaging) from the back of the slide (or chip).

FIG. 3A schematically shows one embodiment of a fluidics system (300),comprising a variety of illustrative reagent and buffer reservoirs incommunication (via tubing (306) or other channeling into a manifoldcomprising valves (305)) with one embodiment of a flow cell (comprisinga side entrance port (301) and one or more heaters 302), wherein thearray or chip (303) is inverted and the exit port (304) is on thebottom, thereby permitting the fluid channel to be drained at least inpart by gravity so that waste can be readily collected into a reservoir.3B shows another embodiment of the system (310), showing the flow cell(311) in relationship to the illumination and optics (312).

FIG. 4 shows a schematic for steps involved in sample preparation (A)and highly parallel sequencing steps (B) for embodiments of theinvention.

FIG. 5 shows a general structure of embodiments of cleavable fluorophorenucleotide conjugates with reversible terminator functionality. The3′-OH group is reversibly blocked by an allyl ether function and thefluorophore is attached via a cleavable allyl carbamate linker (bothshown in frames). After incorporation and signal readout, thefluorophore and the 3-O-allyl protective groups are cleaved by aqueoussolution of Pd (O).

FIG. 6 shows a cleavage mechanism for trimethyl lock based compounds.

FIG. 7 shows a cleavage mechanism for 1,6-rearrangement based compounds.

FIG. 8 is a schematic flow chart for one embodiment of re-phasing.

FIG. 9 shows simulated data showing sequence lead due to incorporationof small amounts of non-terminated nucleotides mixed with the reversiblyterminated nucleotides. Panel A shows actual fluorescent levels, Panel Bshows measured fluorescent levels.

FIG. 10 shows simulated data showing sequence lag due to finiteincorporation efficiency.

FIG. 11 is a chart of an exemplary sequence of extension events using anexemplary 4 templates positions and 3 cycles.

FIG. 12 is a chart of an exemplary sequence of extension events.

FIG. 13 shows a first portion of the chart of FIG. 11.

FIG. 14 shows a second portion of the chart of FIG. 11.

FIG. 15 shows data generated from the iterative application of equations1-3 using parameters in Table 1.

FIG. 16 panel A shows simulated base read data with 10% noise added andlead and lag factors of 1% each, and panel B shows reconstructed datawith the lead and lag removed.

FIG. 17 panel A shows simulated base read data with 10% noise added andlead and lag factors of 1.75% each, panel B shows that attemptedreconstruction is poor as the lead/lag matrix is ill-conditioned, panelC shows reconstructed data with the lead and lag removed for only thefirst 18 bases. The 18-base lead/lag matrix is relatively well behavedand a more precise reconstruction may be performed. Panel D shows a full35 base reconstruction using the product of two identical matrices thatare the inverse of two well-conditioned lead/lag matrices that have leadand lag values set to 0.88% (half of the actual lead and lag values of1.75%).

FIG. 18 shows lead/lag matrix conditions for various lead and lagparameters for a 35 base read. In one embodiment, a condition numberbelow 20 produces accurate reconstruction.

FIG. 19 shows exemplary read length for various values of lead and lag.

FIG. 20 shows synthesis of 3′-O-azidomethyl-dNTPs where the steps denotetreatment with (i) DMSO, AcOH, Ac₂O, 48 h; (ii) SO₂Cl₂, dry CH₂Cl₂, 1-2h; (iii) NaN₃ in DMF, 3 h; (iv) NH₄F in MeOH, 16-20 h; (v) (MeO)₃PO,POCl₃ then (t-Bu₃NH)₄P₂O₇, TEAB, 1 h; vi) NH₄OH.

FIG. 21 shows synthesis of 3′-O-azidomethyl-dGTP where the steps denotetreatment with (i) DMSO, AcOH, Ac₂O, 48 h; (ii) Ph₂NCOCl, DIEA, Pyridine3 h; (iii) SO₂Cl₂, dry CH₂Cl₂, 1-2 h; (iii) NaN₃ in DMF, 3 h; (iv) NH₄Fin MeOH, 24 h; (v) (MeO)₃PO, POCl₃ then (t-Bu₃NH)₃P₂O₇H, TEAB, 1 h; (vi)NH₄OH.

FIG. 22 shows synthetic DNA templates used in exemplary sequencingexperiments. FIG. 22A shows template 20. FIG. 22B shows template 21.FIG. 22C shows template 22. FIG. 22D shows template 23.

FIG. 23 shows the structures of exemplary labeled2,3′-dideoxynucleotides used in the sequencing by synthesis. FIG. 23Ashows these structures for ddUTP and ddCTP. FIG. 23B shows thesestructures for ddATP and ddGTP.

FIG. 24 shows sequencing results using four different 25 nt DNAtemplates. FIG. 24 a shows the results for template 20. FIG. 24B showsthe results with template 21. FIG. 24C shows the results for template22. FIG. 24D shows the results with template 23.

FIG. 25 shows synthesis steps for 2′-fluoro-3′-O-azidomethyl-dNTPs,where the steps comprise the following exemplary conditions (i) DMSO,AcOH, Ac₂O, 48 h; (ii) SO₂Cl₂, dry CH₂Cl₂, 1-2 h; (iii) NaN₃ in DMF, 3h; (iv) NH₄F in MeOH, 16-20 h; (v) (MeO)₃PO, POCl₃ then (t-Bu₃NH)₄P₂O₇,TEAB, 1 h; vi) NH₄OH.

FIG. 26 shows synthesis steps for2′-fluoro-3′-O-azidomethyl-(propargylamino)-dNTP synthesis.

FIG. 27 shows exemplary nucleotide structures with 3′-OH groupprotection that can be cleaved by mild oxidation reactions.

FIG. 28 shows an exemplary general synthetic pathway to install 3′-Oamino hemiacetal group (—CH₂ONH₂) and conversion to nucleotides.

FIG. 29 shows an exemplary synthetic pathway to prepare 3′-0 carbazate(—CH₂ONH₂) nucleotide analogues

FIG. 30 shows an exemplary mechanism of 3′-O amino hemiacetal (—CH₂ONH₂)nucleotides deprotection reaction to generate free 3′-OH group.

FIG. 31 shows an exemplary mechanism of 3′-O-carbazate (—C(O)NHNH₂)nucleotides deprotection. The reaction may be fast due to higher entropycontribution of the leaving molecular nitrogen and carbon dioxide gas.

FIG. 32 shows sequencing by synthesis probe intensity in four channels(blue, green, yellow, and red) for a spot on a chip. Panel A is rawdata, and panel B is data with the color crosstalk removed.

FIG. 33 panel A shows 16-base-long sequence data, and panel B shows thesame data after applying the lead/lag compensation algorithm.

FIG. 34 panel A shows 25-base-long sequence data and panel B shows thesame data after applying the lead/lag compensation algorithm.

FIG. 35 provides examples of chemical structures of the reversiblyterminating nucleotides used in sequencing. These examples include:3′-O-azidomethyl nucleotides, 3′-β-aminoxy nucleotides, 3′-O-allylnucleotides; and disulfide nucleotides.

FIG. 36 provides examples of dyes conjugated to reversibly terminatingnucleotides via a cleavable linker.

FIG. 37 provides examples of compounds useful as cleaving agent“scavengers.”

FIG. 38 shows detection of incorporated nucleotides in an extensionreaction done in the presence of a first scavenger (cystamine).

FIG. 39 shows detection of incorporated nucleotides in an extensionreaction done in the presence of a second scavenger (ATA).

FIG. 40 is a schematic showing one embodiment for the synthesis of3′-O-azidomethyl,7-propargylamido-[3-((2-amidoethyl)dithio)propionamido]-6-carboxy-X-rhodaminedeoxyadenosine triphosphate.

FIG. 41 is a schematic showing one embodiment of a hot embossingtechnique for making slide (or chips) with indentations (which canreceive millions to billions of microbeads comprising nucleic acid).

FIG. 42 is a schematic comparing the structure of natural DNA with DNAthat was labeled with cleavable terminating nucleotides and then thelabel was removed. In this particular schematic, the example showspropargylamino derivatives.

FIG. 43 is a schematic showing a capping step to neutralize the reactivegroups after dye cleavage. For amines, one example that could be used isacetylation (such as acetic acid NHS ester); for the thiols (SH)N-methyl-maleimide or iodoacetamide can be used.

FIG. 44 shows the fluorescence signal from incorporated nucleotideanalogues observed as a function of the composition of the extensionmixture. In this case labeled nucleotides (3′-β-allyl) were supplementedwith up to 1 equivalent of non-labeled terminators (also 3′-O-allyl).The extension was performed and the resulting signal measured. Theresponse is different for different nucleotides tested and is a functionof polymerase bias.

FIG. 45 shows the results for two subsequent extensions performed on 4different DNA template (corresponding to FIGS. 45A, B, C and D,respectively). For extension 1, various amounts of labeled reversibleterminating nucleotides were used (0, 50% and 100%). After cleavage,second extension was performed and the resulting signals were measured(bars on the right in each set). As can be seen the use of 100% labelednucleotides in cycle 1 reduces the signal in subsequent cycle to by 50%compared to non-labeled reversible terminators.

FIG. 46 shows that improvements in sequencing performance can beachieved with a mixture of labeled/unlabeled nucleotides (lower panel)compared to using 100% labeled nucleotides (upper panel). Using such anucleotide mixture results in correct base calls.

FIG. 47 shows the signal decline observed using labeled nucleotides insequencing.

FIG. 48 shows that using a mixture of labeled and unlabeled nucleotides(e.g. a mixture of labeled and non-labeled reversible terminators) andcontrolling extension time can improve performance (e.g. increaseretention of signal) on an automated sequencing device. FIGS. 48A and48B show the results for 15 minute extension (for templates 20 and 23respectively). FIGS. 48C and 48D show the results for 2 minute extension(for the same two templates, respectively). With additional controlprovided (e.g. by reducing extension time from 15 minutes to 2 minutes),the incorporation rate of labeled nucleotides can be controlled andresults in improved fidelity and performance.

FIG. 49 shows that using a mixture of labeled and unlabeled nucleotides(e.g. a mixture of labeled and non-labeled reversible terminators) andcontrolling extension time can improve performance (e.g. increaseretention of signal) on an automated sequencing device. FIGS. 49A, 49B,49C, 49D, 49E, 49F and 49G show the results with 10 minute extension(for templates 20, 30, 21, 31, 22, 32, 23 and 33, respectively). FIGS.49I, 49J, 49K, 49L, 49O and 49P show the results with 1 minute extension(for templates 20, 30, 21, 31, 23 and 33, respectively). With additionalcontrol provided (e.g. by reducing extension time from 10 minutes to 1minute), the incorporation rate of labeled nucleotides can be controlledand results in improved fidelity and performance. Signal retention isdefined as the ratio of signals at the end of the run to the signals atthe beginning of the run.

FIG. 50A shows a cross section view of one embodiment of a flow cellpositioned at a station in fluidic communication with reagent or washsolutions via tubing and inlet/outlet ports.

FIG. 50B is a top view of one embodiment of a flow cell positioned at astation in fluidic communication with reagent or wash solutions viatubing and inlet/outlet ports, the inlet port introducing such solutionsinto the flow cell and the outlet port removing such solutions from theflow cell.

FIG. 51 is a top view of one embodiment of a moving support or carousel,comprising a plurality of small flow cells (in this embodiment, eightflow cells are shown, but there is clearly room for more if desired).

FIG. 52 is a schematic showing a top view of one embodiment of flowcells on a moveable support, in the context of the fluidics provided ateach station.

FIG. 53 is a schematic showing a side view of one embodiment of themoveable support, in the context of both the fluidics and the mechanicsfor moving the support.

FIG. 54 shows a system with no open slots or non-functional positions,wherein the number of flow cells is equal to the number of functionalstations. An associated Table provides the overall time to completesequencing cycles using this system.

FIG. 55 shows a system with no open slots, but non-functional stations,wherein the number of flow cells is greater than the number offunctional stations. An associated Table provides the overall time tocomplete sequencing cycles using this system.

FIG. 56 shows a system with both open slots and non-functional stations.An associated Table provides the overall time to complete sequencingcycles using this system.

FIG. 57 shows a scheme whereby a system with open slots andnon-functional stations can be optimized by movement of flow cells todifferent positions. An associated Table provides the overall time tocomplete sequencing cycles using this system.

FIG. 58 shows one embodiment of a process whereby clockwise andcounterclockwise movement of a carousel permits the repositioning offlow cells into positions for maximum efficiency.

FIG. 59 shows one embodiment of the mechanics for off-loading flow cellsfrom the carousel in a manner that will permit the process of FIG. 58.

FIG. 60 is a schematic (with an associated table) that quantitativelycompares throughput and cycle time for a single flow cell system, a dualflow cell system and an eight flow-cell system.

FIG. 61 is a schematic showing a representative system where stationsare associated with sequencing steps, including some steps where thereis heating.

FIG. 62 is a bar graph showing the relationship of the number of flowcells to cycle time where the flow cells are small.

FIG. 63 is a bar graph showing the relationship of the number of flowcells to cycle time where the flow cells are large.

FIGS. 64A, B and C show one embodiment of a dual camera arrangement forimaging nucleic acid during the sequencing protocol. FIG. 64A showsfirst (FC) and second (SC) cameras in relationship to the LED lightsource (LS) and flow cells (FC). FIG. 64B more clearly show the colorfilters and dichroic beam splitters. FIG. 64C shows the relationship ofthe filters, mirrors and lenses in more detail.

FIGS. 65A and B show one embodiment of a user interface. Morespecifically, FIG. 6A is a picture of the control board (CB) for theinstrument with the computer screen (CS) in the foreground (inelectronic communication therewith). FIG. 65B a picture of a version ofthe software user interface that shows some steps in a protocol (PS) andsome data (simulated) on a chip.

FIG. 66A shows one embodiment of a modified nucleotide comprising anacyclic structure (e.g. acyclic sugar) lacking the (deoxy)ribosefuranose ring. FIG. 66B shows a modified nucleotide comprising thefuranose ring.

DEFINITIONS

To facilitate understanding of the invention, a number of terms aredefined below, and others are found elsewhere in the specification.

In one embodiment, the present invention contemplates nucleic acidsamples or template “arrayed” on a “chip” or other surface or solidsupport. A “chip” is not limited to a silicon chip, but includessupports made of metal, plastic and glass. A “chip” need not be squareand can take a number of shapes. A “chip” may have features (e.g.indentations and the like) or may be smooth. A “chip” is preferablysmaller in dimensions than a conventional microscope slide.

In another embodiment, the nucleic acid to be sequenced is immobilizedon particles or beads and the beads are either in a fixed position ornot in a fixed position in said flow cell. In one embodiment, said beadsare magnetic beads.

In yet another embodiment, nucleic acid to be sequenced is immobilizedon beads that fit into holes or depressions in a “chip” surface.

The term “array” refers to a random or ordered plurality of molecules ona support. Devices, equations, and computer systems for forming andusing arrays of material on a substrate for DNA sequencing are known(e.g., Ju et al., U.S. Pat. No. 6,664,079; Pirrung et al., U.S. Pat. No.5,143,854; Hubbell et al., U.S. Pat. No. 5,71,639; Lipshutz et al., U.S.Pat. Nos. 6,957,149, 5,733,729, 6,066,454, 6,228,593 and 6,546,340; Cheeet al., U.S. Pat. No. 5,795,716; Domnisoru et al., U.S. Pat. No.6,598,013; Schermer et al., U.S. Pat. No. 7,209,836; Gavrilov et al.,U.S. Pat. Application No. 2007/0194249; Eltoukhy et al. In: IEEEInternational Conference on Acoustics, Speech and signal processing,(2006) 2:1032-1035; Margulies et al. (2005) Nature 437:376-380; andGerardo et al. (2008) Nucleic Acids Res. (2008) 36 (4):e25).

The term “plurality” means two or more.

A “subject” can be a human or animal. It is not intended that thepresent invention be limited by the nature or status of the subject,e.g. the human may be disease-free or may have symptoms of a disease.

The term “nucleotide sequence” refers to a polymer comprisingdeoxyribonucleotides (in DNA) or ribonucleotides (in RNA).

The term “interrogation position” when made in reference to a nucleotidesequence refers to a location of interest in the sequence, such as thelocation at which the identity of a nucleic acid is sought to bedetermined.

The term “preceding nucleic acid” when made in reference to a firstnucleic acid in relation to a second nucleic acid that is located at aninterrogation position in a nucleotide sequence refers to a nucleic acidthat is inserted during synthesis into the nucleotide sequence beforethe insertion of the second nucleic acid at the interrogation position.The term “subsequent nucleic acid” when made in reference to a thirdnucleic acid in relation to the second nucleic acid at the interrogationposition refers to a nucleic acid that is inserted during synthesis intothe nucleotide sequence after the insertion of the second nucleic acidat the interrogation position.

The terms “probe” and “label” are interchangeably used to describe achemical moiety that, when attached to a composition of interest, actsas a marker for the presence of the composition of interest. Probes areexemplified by fluorescent moieties such as 5-carboxyfluorescein,6-carboxyrhodamine-6G, N,N,N′,N′-tetramethyl-6-carboxyrhodamine, and6-carboxy-X-rhodamine. Probes also include a fluorescence energytransfer tag that comprises an energy transfer donor and an energytransfer acceptor. The energy transfer donor is exemplified by5-carboxyfluorescein and cyanine, and the energy transfer acceptor isexemplified by dichlorocarboxyfluorescein,dichloro-6-carboxyrhodamine-6G,dichloro-N,N,N′,N′-tetramethyl-6-carboxyrhodamine, anddichloro-6-carboxy-X-rhodamine. The mass tag is exemplified by a2-nitro-a-methyl-benzyl group, 2-nitro-a-methyl-3-fluorobenzyl group,2-nitro-a-methyl-3,4-difluorobenzyl group, and2-nitro-a-methyl-3,4-dimethoxybenzyl group.

The term “probe corresponds to a nucleotide” means that the probe servesas a marker for the presence of the nucleotide. Thus, detecting thepresence of the probe also detects the presence of the nucleotide.

The term “field flattening” when in reference to pixel intensity of animage refers to reducing differences in pixel intensity between two ormore pixels at different spatial locations on the image of a uniformlyradiating surface.

The terms “reducing,” “decreasing” and grammatical equivalents when inreference to the level of a molecule and/or phenomenon (e.g., lightintensity, chemical concentration, correlation between two event, etc.)in a first sample relative to a second sample, mean that the quantity ofmolecule and/or phenomenon in the first sample is lower than in thesecond sample by any amount that is statistically significant using anyart-accepted statistical method of analysis. In some embodiments, thequantity of molecule and/or phenomenon in the first sample is at least10% lower than, at least 25% lower than, at least 50% lower than, atleast 75% lower than, and/or at least 90% lower than the quantity of thesame molecule and/or phenomenon in a second sample. The term “reducing”includes, but does not require, a 100% lowering in the quantity of themolecule and/or phenomenon in the first sample compared to the secondsample.

The terms “increasing,” “elevating” and grammatical equivalents when inreference to the level of a molecule and/or phenomenon (e.g., lightintensity, chemical concentration, correlation between two event, etc.)in a first sample relative to a second sample, mean that the quantity ofmolecule and/or phenomenon in the first sample is higher than in thesecond sample by any amount that is statistically significant using anyart-accepted statistical method of analysis. In some embodiments, thequantity of the molecule and/or phenomenon in the first sample is atleast 10% greater than, at least 25% greater than, at least 50% greaterthan, at least 75% greater than, and/or at least 90% greater than thequantity of the same molecule and/or phenomenon in a second sample.

“Spectral” is a term that refers to electromagnetic radiation. In oneembodiment, the electromagnetic radiation is in the visible light region(wavelength of approximately 400-700 nanometers), such as that emittedby fluorescent moieties.

The terms “spectral filter” and “color filter” are interchangeably usedto refer to a filter for detection of a particular range ofelectromagnetic wavelengths, such as in the visible region, thereby Theterms “spectral crosstalk” and “color crosstalk” refer to any phenomenonby which a spectral signal, or a digital signal that corresponds to aspectral signal, that is transmitted and measured in one channel oftransmission creates an undesired effect in another channel. Forexample, spectral crosstalk may occur when exciting only a green dye,resulting in a signal that is visible in the yellow channel as well asin the green channel. Using methods disclosed herein, if this spectralcrosstalk is calibrated, it may be removed from subsequent measurementseven if the dyes are mixed in unknown quantities.

The term “low pass filter” refers to a filter that passes slowlyspatially varying intensity signals but reduces signals with higherspatial variation than a desired cutoff value. Exemplary software forcarrying out these steps is shown Appendix C, which is a source code forcreating a flat map calibration image.

The term “computer readable medium” refers to a medium, such as acompact optical disc, that is used to store and retrieve digital data.

One element is in “fluid communication” or “fluidic communication” withanother element when it is attached through a channel, tube or otherconduit that permits the passage of liquid, gas, vapor and the like.“Tubing” can be made of a variety of materials, including put notlimited to various plastics, metals and composites. Tubing can be rigidor flexible. Tubing can be “attached” in a detachable mode or a fixedmode. Tubing is typically attached by sliding into or over (both ofwhich are examples of “slidably engaging”) other tubing or connectors.

In some embodiments, certain elements are in electronic communicationwith other elements (and thereby “communicate electronically”). Forexample, the user interface is in electronic communication with theprocessor. “Electronic communication” can be implemented in a hard-wiredelectrical connection, e.g., a shielded cable, or an optical connection,e.g., an optical fiber, a wireless communication, e.g., infrared orradiowaves, a combination thereof, and the like.

Sequencing typically involves a number of steps (e.g. steps a through e)that are repeated. For this reason, each repeat of the collection ofsteps is often called a “cycle.” The steps in a cycle typically have atemporal order, with a first step and a last step, and some stepsperformed after said first step and before said last step. Not all stepsin a cycle need be reaction steps. In one embodiment, one or more stepsmay comprise pausing at a “dwell” station (e.g. where a flow cell ismoved to a position where no new reagents are added). The “cycle time”is the time needed to complete one cycle. In sequencing by synthesis,each cycle identifies or “reads” one base at all or some of the variousspots on the chip.

In one embodiment, the present invention contemplates positions being ina “fixed relationship” on a moveable support, in the sense that themarkings on a wall clock are in a fixed relationship. Thus, flow cellsmay be put in positions such that a first and second flow cell are in afixed relationship (e.g. the first flow cell at 1 o'clock and the secondflow cell at 2 o'clock); but this fixed relationship may be changed bymoving the first and/or second flow cell to new positions, therebycreating a new fixed relationship.

DESCRIPTION OF THE INVENTION

For further clarity, the invention is described below under thefollowing headings

A. Sequencing By Synthesis; B. Device Embodiments and Elements; C.Nucleotides; D. Reducing Lead And Lag; E. Dephasing; F. FieldFlattening; G. Spot Location in the Array; H. Image Sharpening; I. SpotBrightness Determination; J. Neighbor Influence Elimination; K. SpectralCrosstalk Calibration; L. Base Calls; and M. Software Appendices A-C

A. Sequencing by Synthesis

The invention relates to methods and compositions for determining theidentity of nucleic acids in nucleotide sequences using, for example,data obtained from sequencing by synthesis methods. Methods of DNAsequencing are generally described in Metzker, Genome Res. (2005)15(12): 1767-1776 and Shendure et al. (2004) Nature Reviews Genetics 5:335-344. The Sanger sequencing method or chain termination or dideoxymethod is a technique that uses an enzymatic procedure to synthesize DNAchains of varying length in different reactions that contain dilutedconcentrations of individual dideoxy nucleotides mixed in with normalnucleotides. DNA replication is stopped at positions that are occupiedby one of the dideoxy nucleotide bases resulting in a distribution ofnucleotide fragments since the normal nucleotides will properlyincorporate. Unnatural ddNTP terminators replace the OH with an H at the3′-position of the deoxyribose molecule and irreversibly terminate DNApolymerase activity. The resulting fragment lengths are determined todecipher the ultimate sequence. Electrophoretic separation of thedeoxyribonucleotide triphosphate (dNTP) fragments may be accomplishedwith single-base resolution.

In sequencing by synthesis, nucleotides conjugated with fluorescentmarkers that incorporate into a growing double-stranded nucleic acidfrom the single strand are detected. For example, one may immobilizetemplate DNA on a solid surface by its 5′ end. One may accomplish thisby annealing a sequencing primer to a consensus sequence and introducingDNA polymerase and fluorescent nucleotide conjugates (alternatively, aself-priming hairpin can be introduced by PCR or ligation to thetemplate). One detects nucleotide incorporation using a laser microarrayscanner or fluorescent microscope by correlating a particularfluorescent marker to a specific nucleotide. After each nucleotide isincorporated and the fluorescent signal is detected, one bleaches orremoves the fluorescent moiety from the nucleotide conjugate so as toprevent the accumulation of a background signal.

In one embodiment, the present invention contemplates DNA sequencing bysynthesis using an automated instrument, as well as methods andcompositions useful for sequencing using such an instrument. In oneembodiment, the instrument comprises a flow cell (FIGS. 2A and 2B) withat least two fluidics ports, a substrate with sequenceable nucleic acidmolecules attached to the substrate, reagent and waste reservoirs andfluidic system connecting the reservoirs to the flowcell (FIG. 3). Theflowcell is interfaced with a detection system to monitor theincorporation of the nucleotides.

As noted above, in a preferred embodiment, the present invention employsa plurality of smaller flow cells (with an area for imaging ofapproximately 35 mm×2.5 mm, or less) mounted on a support, in the mannerof a carousel. FIG. 51 shows such one embodiment of such a carousel (33)comprising eight such flow cells (5) mounted on the surface (30), eachflow cell associated with a station (31), each station associated with astep (or partial step) in the sequencing protocol. The carousel (33) canrotate clockwise or counterclockwise in order to bring each flow cell(5) to the next station (31) for processing. In a preferred embodiment,the carousel (30) transports one or more flow cells (5) to stationblocks (FIG. 50A, element 15), at various station locations (31) isshown. Generally, the transport occurs in short indexing motions, sothat the flow cells spend most of the time at station locations ratherthan in motion. While a plurality of flow cells is preferred, theprocessor of the system is programmable, such that a user may run fromone to the maximum number of flow cells.

FIG. 50A shows (in cross section) a single, removeable flow cell (5)positioned at a station (and aligned with a station block 15) in fluidiccommunication with reagent or wash solutions via tubing (16) associatedwith an inlet port (20) and tubing (17) associated with an outlet port(21). In one embodiment, the flow cell is constructed of at least threepieces including a base plate (10), a spacer film (13) and a top cover(11). The samples to be sequenced may either be located on a separatechip affixed to the base plate, the base plate itself, or the bottomsurface of the top cover. An “array,” “slide” or “chip” refers to any ofthe aforementioned locations where the samples to be sequenced arelocated. The top cover (11) is preferably transparent so that a chippositioned inside the flow cell can be imaged. The flow path (12) is anarrow space between the base plate (10) and the top cover (11) andforms the area where the reagents and wash fluids may flow across thesamples on the chip from the flow cell inlet (20) to the flow celloutlet (21). The station block (15) also has ports that approximatelyline up with the flow cell inlet (20) and outlet (21). Elastomericseals, such as O-ring seals, seal the interface of the two sets of portsso that fluids may flow between the flow cell and station block withoutleaking. In stations which deliver reagents, the seals (18) have asmaller open area where the fluids may flow than the seals (19) instation where wash fluids flow. Thus, any reagents trapped in sealingareas at reagent stations are more efficiently washed from the bottomsurface of the flow cell bottom plates (10) at the wash stations so asto not contaminate the flow cell with reagents from previous steps.Reagents or wash fluids are drawn through the station blocks (15) andflow cells (5) by fluid in tubing (17) drawn by a pump, such as asyringe pump (FIGS. 52 and 53, element 41). Reagents are suppliedthrough tubing (16) from reagent vessels or reservoirs (FIG. 52, element43). Stations block may be temperature controlled by resistive heatingor thermoelectric heating/cooling elements (22A) that are incommunication with a power source (not shown) via wires (22B) or otherconnections. Since the flow cell connection to reagents and washes is,in a preferred embodiment, right at (i.e. in the immediate vicinity of)the flow cell rather than through a few feet of tubing back to a valve,there is less reagent waste and less washing necessary to clean out thewetted materials between steps.

FIG. 50B is a top view of one embodiment of a removeable flow cell (5)positioned at a station in fluidic communication with reagent or washsolutions via tubing and inlet (20) and outlet (21) ports, the inletport (20) introducing such solutions into the enclosed flow chamber (12)defining the flow path of the flow cell (5) and the outlet port (21)removing such solutions from the flow cell (5). The shape of the spacerfilm (13) which forms the periphery of the flow path (12) may be seen.The base plate (10) may extend beyond the bounds of the spacer film (13)and the top cover (11), or it may be flush with the edges. An extendeddimension may be used to clamp the flow cell to the station block (15),without putting pressure on the more delicate and thin material thatmakes up the top cover (11). In this manner, the flow cell (5) is heldin position by fastening means (13) which can be a clamping device or anadhesive, including sticky tape (preferably double sticky tape).

FIG. 52 shows one embodiment of a fluidics configuration for the stationblocks (15) positioned around and under a carousel (30). In thisembodiment, seven station blocks (15) are connected (although more orfewer can be used in other embodiments) to one or more source bottles orreservoirs (43) through tubing (16) and at least one waste container(40) through tubing (17) connected to a syringe pump (41). One stationblock is shown with no fluid connections, since in certain embodiments,one or more stations may not require fluids (for example, at an imagingstation, at a dwell station, or at a load/unload station). All of thestations with fluid connections are shown to be in fluid communicationvia tubing (17) with at least one syringe pump (41) through amultiplexing valve (42). In other embodiments, more than a singlesyringe pump may be used. In one embodiment, at least one wastereceptacle (40) is also connected to the syringe pump to receive usedreagents and wash fluids from the stations. In some embodiment,different waste bottles are used for different used reagents, in orderto permit certain reagents to be collected and used again.

In one embodiment, the carousel (30) carries/transports the flow cells(5) to different locations, some of these locations being associatedwith stations, at least some of these stations being reagent deliverystations. In one embodiment (FIG. 53), the flow cells (5) are movedbetween the station blocks (15) using a rotary actuator (46). In thecase where the flow cells are positioned at a reagent delivery station,the flow cells (5), prior to movement, must be separated/disconnectedfrom the station blocks (15). In one embodiment, this is done by raisingthe carousel (30). In another embodiment, this is done by lowering thestation blocks (15). In yet another embodiment, a combination of raisingand lower is performed. A linear actuator (47) is shown and may be usedto raise the rotary actuator (46), which in turn raises the flow cells(5) on the carousel (30) by raising the carousel. In one embodiment,check valves (45) are used in the supply lines (16) between the reagentvessel (e.g. reservoir) (43) and the station blocks (15), so that whenthe continuous fluid column is broken by raising the flow cells (5) fromthe station blocks (15), the fluid does not flow back into the reagentbottles (43). The fluid level (FL) in the reagent bottles (43) ispreferably kept below the vertical level of the station blocks (15), sothat in the case of a valve failure, fluid does not continuously flowout of the reagent bottles (43), and flood the system when the flowcells (5) are raised from the station blocks (15). The actuators (46 and47) are in electronic communication with a programmable processor, whichis in turn connected to a user interface (see FIGS. 65A and 65B). Thesystem thereby permits multiple flow cells run in parallel at differentstages in the sequencing protocol.

In one embodiment, flow cells are moved from their position on thecarousel to a different position. This can have advantages, particularlywhere there are “open” slots on the carousel (i.e. the position lacks aflow cell) and/or non-functional stations (or dwell stations). Tounderstand this advantage clearly, it is helpful to first consider thesituation where there are no open slots or non-functional stations. FIG.54 shows a system with seven functional stations, numbered one throughseven, equally positioned around a carousel with seven positions fordetachable flow cells, numbered A through G. The associated table showsthe progression of flow cells through the seven stations for a systemthat indexes every five minutes. It takes 105 minutes for flow cell A tocomplete three cycles around the system. By contrast, FIG. 55 shows asystem with seven functional stations, numbered one through seven, andthree non-functional stations (e.g. assuming a sequencing cycle requiresonly seven steps), numbered eight through ten, equally positioned arounda carousel with ten positions for detachable flow cells, numbered Athrough J. The table shows the progression of the ten flow cells throughthe ten stations for a system that indexes every five minutes. Becauseof the three non-functional stations, it takes 150 minutes for flow cellA to complete three cycles around the system. FIG. 56 shows yet anotherproblematic situation where seven functional stations, numbered onethrough seven, and three non-functional stations, numbered eight throughten, are equally positioned around a carousel with ten positions fordetachable flow cells—but where only seven flow cells, numbered Athrough G, populate the carousel (i.e. there are three open slots). Thetable shows the progression of flow cells through the ten stations for asystem that indexes every five minutes. It takes 150 minutes for flowcell A to complete three cycles around the system. Because the system isa synchronous indexing system (and assuming there is no means for movingflow cells to different positions), the flow cells all spend fiveminutes indexing through each of the ten stations, even thought some arenon-functional.

Having now described the problem, the present invention contemplates inone embodiment a solution comprising a mechanism to bypassnon-functional stations and/or open slots. FIG. 57 shows a system withseven functional stations, numbered one through seven, and threenon-functional stations, numbered eight through ten, equally positionedaround a carousel with ten positions for detachable flow cells. Onlyseven flow cells, numbered A through G, populate the carousel. Thesystem is fitted with a mechanism (shown in more detail in FIG. 59) thatcan transfer flow cells from position 7 to any location on the carousel.The table shows the progression of flow cells through the ten stationsfor a system that indexes every five minutes. This is a solution to theproblem, because the flow cell transfer mechanism can move flow cellsthat have completed a cycle at position 7 to a location on the carouselwhich will bring them to position 1 at the next cycle, thereby bypassingthe non-functional positions. By moving a flow cell to a differentposition, it takes only 105 minutes for flow cell A to complete threecycles around the system.

FIG. 58 shows one embodiment of how the system can bypass stationsand/or open slots, utilizing both clockwise and counterclockwise motionof the carousel. FIG. 58 shows a sequencing system with seven functionalstations (labeled 1 through 7) and three non-functional stations(labeled 8 through 10) equally spaced around a circular carousel. Thesystem is configured to process flow cells on an under populatedcarousel as fast as possible as shown in FIG. 57, wherein non-functionalstations may be bypassed. Under a normal indexing operation, thecarousel turns clockwise one station, thereby advancing all the flowcells to the next station required for processing. One station,preferably the last functional station (number 7 in this case), isfitted with a device that can remove and replace flow cells from thecarousel (e.g. off-load them onto a platform). In one embodiment, thisis a detection station where flow cells are removed from the carouseland placed on a programmable positioning table so that they may bescanned by a detector, as shown in FIG. 59. In this case, the samemechanism that is used to place the flow cells on the station 7positioning table also has the capability to replace the flow cell toany carousel position that is located at position 7. In FIG. 58 a, eachof the ten positions on the carousel where flow cells may be located islabeled 1′ through 10′. In the figure, seven flow cells to be processed,labeled A through G, are located at corresponding carousel positions 1′through 7′. Three carousel positions, 8′, 9′ and 10′, are empty. FIG.58A shows flow cell G being removed from the carousel onto a remotepositioning table for scanning of samples. Rather than a normalclockwise carousel indexing motion at the completion of the cycle, thecarousel is moved counter clockwise so that position 10′ is in front ofstation 7, FIG. 58 b. The flow cell G is replaced onto the carousel atposition 10′, FIG. 58 c. The carousel is then moved clockwise to thenext indexing position so that each of the flow cells A through F areposition to the next station in their sequence and flow cell G ispositioned at station 1, bypassing all of the non-functional stations,FIG. 58 d. FIG. 58 shows only one example of an underpopulated carousel.It should be understood that any number of flow cells on the carouselmay be processed and moved in a similar manner.

FIG. 59 shows one embodiment of a detection station and the mechanicsfor moving flow cells. In this embodiment, the carousel (30) rotates theflow cell (5) in front of the detection station. The station comprises avertical actuator (50) fitted with a gripper (51) that can grasp theflow cell (5) and lift it off the carousel (30). A programmablepositioning table (53) is then positioned under the lifted flow cell (5)by a linear positioner (54). The positioner (54) is also used toposition various parts of the flow cell under the detector (52) sohigh-magnification data may be generated across the entire desired areaof the flow cell (5). By using the carousel motor (46) to position anylocation around the carousel (30) in front of the detection station, itis possible to replace/reposition the flow cell (5) to any location onthe carousel.

In one embodiment, the sequencing by synthesis is carried out usingreversibly terminating nucleotides and polymerase. The nucleotidescomprise a protective group at their 3′-OH which prevents multipleincorporations and allows for accurate decoding of the sequence. Onceincorporated the protective groups can be cleaved with high efficiencyand specificity to allow subsequent nucleotide incorporations. Thenucleotides may also comprise a detectable label which can be cleavedafter the detection.

In one embodiment, the present invention contemplates a series of methodsteps, which an instrument for automated sequencing by synthesis maycarry out. In one embodiment, the process is comprised of the followingreagent reservoirs: 1) Extend A (reversibly terminated labelednucleotides and polymerase); 2) Extend B (reversibly terminatedunlabeled nucleotides and polymerase, but lacking labeled nucleotideanalogues); 3) Wash solution 1 (e.g. in one embodiment comprising adetergent, such as polysorbate 20, in a citrate solution, such as salinesodium citrate); 4) Cleave solution; 5) Wash solution 2 (e.g. in oneembodiment, comprising a detergent, such as polysorbate 20 in a buffercomprising tris(hydroxymethyl)aminomethane or “Tris”). Of course, thepresent invention is not limited to particular concentrations ofreagents in these solutions (and other buffers and detergents can beemployed). Nonetheless, in order to achieve high throughput rates, theincorporation reactions and the cleavage reactions are desired to befast. In one embodiment, high reaction rates are achieved by increasingthe concentration of reagents, agitation, pH or temperature (or thecombination of all these factors). The incorporation rate in addition isdependent on the specific activity and processivity of the polymeraseused. In one particular embodiment (which is provided by way of anon-limiting example), the reagents solutions have the followingcompositions and concentration ranges:

1) Extend A—reversibly terminated labeled (1 nM to 1 uM) and non-labelednucleotides (1 uM to 100 uM) and a first polymerase (1-500 ug/ml)); 2)Extend B—reversibly terminated non-labeled nucleotides (1 uM to 100 uM)and a second polymerase (1-500 ug/ml)); 3) Wash solution 1 (3×SSC, 0.02%Tween 20); 4) Cleave solution (50-100 mM TCEP); 5) Wash solution 2 (100mM Tris-HCl, 0.02% Tween 20, 10 mM KCl, 20 mM (NH₂)₂SO₄. In oneembodiment, the first polymerase incorporates labeled nucleotides betterthan the second polymerase, which incorporates unlabeled nucleotidesmore efficiently. Examples of commercially available polymerases thatcan be used include Therminator I-III. These polymerases are derivedfrom Thermococcus sp. and carry mutations allowing for incorporation ofmodified nucleotides. Examples of these polymerases are listed in Tablebelow:

Therminator I NEB cat. # 9° N A485L (exo-) M0261L DNA PolymeraseTherminator II NEB cat. # 9° N A485L/Y409V (exo-) M0266L DNA PolymeraseTherminator III NEB cat. # 9° N L408S/Y409A/P410V (exo-) M0333L DNAPolymeraseOther polymerases derived from 9 deg N parent polymerase or Thermococcussp. could also be used. Other suitable polymerase families couldconceivably be used after introducing mutation controlling the stericgate and enabling reversible terminators incorporation.

In one embodiment, the sequenceable DNA (preferably loaded on the chipor slide) is subjected to these solutions and compositions in theinstrument, and the sequencing is performed using automated protocol.Again, it is not intended that the present invention be limited to aprecise protocol or series of method steps. The order and number ofsteps can vary, as well as the time taken for each step. By way of anon-limiting example, in one embodiment, the instrument protocolcomprises (and is configured) as follows:

1. Extend A—0.5-5 minutes (delivery+agitation)

2. Extend B—1-20 minutes (delivery+agitation)

3. Wash 2—5-10 minutes (10-20×delivery and agitation followed by flowcell evacuation)

4. Image

5. Cleave A (or Cleave A and B)—1-5 minutes (delivery+agitation)

6. Wash 1—5-10 minutes (10-20×delivery and agitation followed by flowcell evacuation)

7. Wash 2—5-10 minutes (10-20×delivery and agitation followed by flowcell evacuation)

8. Go to step 1

The cycle may be repeated as desired and images may be taken andsubsequently analyzed to decode the DNA sequence present at eachlocation. As noted above, in one embodiment, one or more of these stepsis associated with a “station” (having the requisite reagent or washdelivery elements) and flow cells are moved to each station in order tocarry out the step of the sequencing protocol. Any one of these stepscan be done at two stations if desired, i.e. a step taking a longer timecan be completed over the course of two stations, each station doing apart (e.g. half of the step).

In one embodiment of the above indicated cycle, eight nucleotideanalogues are employed: four (A, T, C, G) cleavably labeled andreversibly terminated; four (A, T, G, C) unlabeled but reversiblyterminated. In one embodiment, the concentration of the labeledanalogues is at a relatively low concentration [e.g. just enough to beincorporated into a substantial portion (e.g. so the label is visibleand detected) of the plurality of primers, whether they be detachedprimers or self-priming hairpins on the template]. By contrast, theunlabeled analogues, in one embodiment, are employed in a relativelyhigh concentration (e.g. in order to drive the extensions to completion,and avoid primers, whether they be detached primers or self-priminghairpins, that lack incorporation of a first nucleotide analogue). Ithas been found empirically that the use of unlabeled nucleotidesimproves read lengths, and reduces lead and lag (discussed below).

One example of a currently optimized protocol is shown in the Table(below),

Nucleotide Labeled nucleotides [nM] Un-labeled nucleotides [nM] dCTP 30250 dATP 20 250 dGTP 30 250 TTP 30 250wherein un-labeled nucleotides are employed in ratios between 8.33 to 1and 12.5 to 1 (relative to labeled nucleotides). In one embodiment, thelabeling (i.e. incorporation) step uses Kapa RevTerm polymerase (fromKapa Biosystems, Woburn, Mass.) at 2 μg/ml and is performed at 55 deg C.for 1-2 minutes. This is followed by synchronization step where onlynon-labeled nucleotides are used at 25 μM concentration and a polymerasederived from 9 deg N (Thermococcus sp). at 25 μg/ml is used. This stepis also carried out at 55 deg C. Thus, unlabeled nucleotide analoguescan be employed together with labeled nucleotides, as well as in stepswhere no labeled nucleotides are employed.

B. Device

In one embodiment, the present invention contemplates using an opticalsystem, for exciting and measuring fluorescence on or in samplescomprising fluorescent materials (e.g., fluorescent labels, dyes orpigments). In a further embodiment, a device is used to detectfluorescent labels on nucleic acid. In another embodiment, the devicecomprises a fluorescent detection system and a flow cell for processingbiomolecules (e.g., nucleic acid samples) arrayed on a “chip” or othersurface (e.g., microscope slide, etc.). The flow cell(s) permit the userto perform biological reactions, including but not limited to,hybridization and sequencing of nucleic acids. Using a plurality ofsmaller flow cells moving to stations associated with steps (or partialsteps) of the sequencing protocol permits the user to perform smallersequencing jobs (100 megabases to 1 gigabases) without wasting reagentor waiting long times to get access to a shared machine.

It is not intended that the present invention be limited to particularlight sources. By way of example only, the system can employultra-bright LEDs (such as those available from Philips LumiledsLighting Co., San Jose, Calif.) of different colors to excite dyesattached to the arrayed nucleic acids. These LEDs are more costeffective and have a longer life than conventionally used gas or solidstate lasers. Other non-lasing sources of lights such as incandescent orfluorescent lamps may also be used.

FIG. 1 shows a useful configuration of the LEDs, whereby the emittedlight converges on a region or platform (e.g., suitable for positioningthe flow cell or sample). However, linear arrays of LEDs can also beused.

It is not intended that the present invention be limited to particularlight collection devices. By way of example only, the system may employa high sensitivity CCD camera (such as those available from RoperScientific, Inc., Photometric division, Tucson Ariz. or those availablefrom Apogee Instruments, Roseville, Calif.) to image the fluorescentdyes and make measurements of their intensity. The CCD cameras may alsobe cooled to increase their sensitivity to low noise level signals.These may also be CMOS, vidicon or other types of electronic camerasystems.

Since LED illumination light is not a collimated beam as from lasers, itis therefore an appropriate choice for imaging a larger area of manynucleic acid spots. To get sufficient light and therefore fluorescentsignals over the larger area, the area seen by each pixel of the cameramust be of sufficient size to allow enough fluorescent dye molecules tocreate a sufficient signal (for example, an Apogee U13 CCD available has1.3 megapixels of 16 microns in size, while the Apogee U32 has 3.2megapixels of 6.8 microns in size).

To increase capacity and efficiency, the present invention contemplatesin one embodiment, a two flow cell system (e.g. while one chip in afirst flow cell is undergoing one or more reaction steps, a second chipin a second flow cell is being scanned and imaged) with a single camera.In yet another embodiment of an imaging system, two flow cells and twocameras are employed.

In one embodiment, the chip containing the array of nucleic acid spotsis processed in a transparent flow cell incorporated within theinstrument, which flows reagent past the spots and produces the signalsrequired for sequencing (see FIGS. 2A and 2B). In a particularembodiment, the chip remains in the flow cell while it is imaged by theLED detector. The flow cell and associated reagents adds the nucleicacids, enzymes, buffers, etc. that are required to produce thefluorescent signals for each sequencing step, then the flow celldelivers the required reagents to remove the fluorescent signals inpreparation for the next cycle. Measurement by the detector occursbetween these two steps. In order for reactions to take place, the flowchannels are configured to be of sufficient dimensions. For example, inone embodiment, the flow-cell fluid channel formed by the array and theflat surface of the flow cell are at least 0.1 mm in depth (moreparticularly 0.5 mm in depth) and the volume formed by the chip, theblock and the seal is at least 100 microliters in volume (moreparticularly, between 100 and 700 microliters, and still moreparticularly, between 150 and 300 microliters, e.g. 200 microliters, involume). Where a plurality of smaller flow cells is used, in oneembodiment, the volume of reagents introduced into the flow cell isapproximately 50 microliters or less, typically approximately 20microliters.

In one embodiment, the flow cell is motionless (i.e., not moved duringreactions or imaging). On the other hand, the flow cell can readily bemounted on a rotary or one or more linear stages, permitting movement.For example, in a two flow cell embodiment, the two flow cells may moveup and down (or side to side) across the imaging system. Movement may bedesired where additional processes are desired (e.g., where exposure toUV light is desired for photochemical reactions within the flow cell,such as removal of photocleavable fluorescent labels), when multipleflow cells share a single camera, or when the field of view of thedetection system is smaller than the desired area to be measured on theflow cell. The detector system may also be moved instead of or inaddition to the flow cell.

In a further embodiment, the flow cell is in fluid communication with afluidics system (see illustrative system shown in FIG. 3. In oneembodiment, each bottle is pressurized with a small positive gaspressure. Opening the appropriate valve allows reagent to flow from thesource bottle through the flow cell to the appropriate collectionvessel(s). In one embodiment, the nucleotides and polymerase solutionsare recovered in a separate collection bottle for re-use in a subsequentcycle. In one embodiment, hazardous waste is recovered in a separatecollection bottle. The bottle and valve configuration allow the washfluid to flush the entire valve train for the system as well as the flowcell. In one embodiment, the process steps comprise: 1) flushing thesystem with wash reagent, 2) introducing nucleotides (e.g. flowing anucleotide cocktail) and polymerase, 3) flushing the system with washreagent, 4) introducing de-blocking reagent (enzyme or compounds capableof removing protective groups in order to permit nucleic acid extensionby a polymerase), 5) imaging, 6) introducing label removing reagent(enzyme or compounds capable of removing fluorescent labels), and 7)flushing the system with wash reagent. In one embodiment of the presentinvention, the steps described above can be achieved by moving a flowcell to different ports/locations.

In the embodiment with moving flow cells (described above), the fluidpath from the source reagent bottles (or reservoirs) to the pump isintermittently broken by the disconnecting of the flow cells from thestation (prior to the next movement). Were the fluid allowed to returnto the reagent bottles, the fluid would need to be pumped back throughthe tubing prior to filling a flow cell, thereby taking more time.Therefore, in a preferred embodiment, a check (or other type of) valvemay be used to assure that fluids to not flow out of the tubing back tothe reagent bottles. This approach is also useful, given the precisevolume may be unknown since a full bottle would start with the fluidlevel in the dip tube closer to the flow cell than a nearly emptybottle.

Having the reagent sources (e.g. reservoirs) below the flow cell servesas a backup strategy. In this regard, should the check valve asdescribed above fail for any reason, then fluids in the tubing wouldflow back to the source bottles rather than out through the stationports and onto the bottom of the instrument.

It is also important, in the moving flow cell embodiment, that fluid maybe maintained in the flow cell (when desired) after it is disconnectedfrom the station ports and while it is being moved from one position(“station”) to another. This can be achieved by a number of approaches.For example, this can be achieved with a flow cell design that does notallow fluid to entirely exit due to horizontal orientation, as well asdue to small dimensions, which allows the viscosity of the fluids tohelp keep them stationary relative to the flow cell. Alternatively, thiscan be achieved with valving within the flow cell.

The mechanics of connecting and disconnecting flow cells at each stationcan be done in a number of ways. In a preferred embodiment, the presentinvention contemplates a reagent source (e.g. reservoir) on one port,and a pump on other side, automatically detachable from both connectionsprior to movement of the flow cell.

In one embodiment, the present invention contemplates systems, methodsand instruments employing two cameras. As shown in FIG. 62A, each of thecameras is dedicated to measuring two of the four colors in the system.For example, while camera number 1 is measuring the blue and yellowchannel, camera number 2 is measuring the green and red channels. In oneembodiment, the cameras are used with 28× magnification. Where two 5megapixel un-cooled CCD cameras are used, very high resolution lensesare preferred. A further advantage to using the pair of cameras is thatthe CCD chip is significantly smaller in the 5 megapixel camera (11 mmvs. 51 mm diagonal) as compared to higher pixel-count camera chips,which leads to greater uniformity of image illumination and less opticaldistortion at the image edges.

In one embodiment, more than one color is illuminated at a time. Mostfluorescent microscopes that use epi-illumination (light follows thesame path as the image through the objective) illuminate only one colorat a time. By contrast, in a preferred embodiment (FIGS. 64A, 64B, and64C) of the dual camera system, two different fluorescent dyes aremeasured simultaneously by the two cameras. In one embodiment, dichroicmirrors are used to split out the long wavelength light from the shorterwavelength light. FIGS. 64B and 64C show the filters, mirrors andoptical path for one embodiment of a dual camera system.

In a preferred embodiment, LED (light emitting diode) illumination isused instead of the typical laser or incandescent white lightillumination. Within just the past few years, breakthroughs in LEDtechnologies have given rise to ultra-bright LED systems. For example,Luminus Devices (Billerica Mass.) makes LEDs that are nearly an order ofmagnitude brighter than those available just three years ago. They areavailable in blue, green and red (and an amber LED will be launched inearly 2010). Spectra for these are close enough to the excitationspectra for the preferred four dyes (Alexa 488, Cy3, Texas Red and Cy5)used for labeling the nucleotide analogues.

LEDs are far superior to incandescent systems, since their lifetime ismeasured in tens of years rather than months. It used to be thatconventional white light incandescent systems were able to put morelight through an epi-illumination system than an LED based system. Nowthe opposite is true. Depending on the color, one is able to get 2 to 5times the amount of light onto the nucleic acid sequencing chip ascompared to a current white light system.

The system can be made to include a user interface system. The Labview(National Instruments, Austin, Tex.) system is available and providessoftware for computer controlled systems. Galil Motion Control (Rocklin,Calif.) provides motion control systems that can be interfaced tocontrol the instrument. FIG. 65A shows a control board (CB) of thesystem in communication with a computer screen (CS) which serves as auser interface, wherein the interface is in electronic communicationwith the computer processor (CP). FIG. 65B shows a closeup of thecomputer screen, allowing for the control over reagents and thesequencing protocol steps (PS).

C. Nucleotides

The invention's compositions and methods contemplate using nucleotidesequences that contain nucleotides. The terms “nucleotide” and “nucleicacid” refer to constituents of nucleic acids (DNA and RNA) that containa purine or pyrimide base, such as adenine (A), guanine (G), cytosine(C), uracil (U), or thymine (T)), covalently linked to a sugar, such asD-ribose (in RNA) or D-2-deoxyribose (in DNA), with the addition of fromone to three phosphate groups that are linked in series to each otherand linked to the sugar. The term “nucleotide” includes nativenucleotides and modified nucleotides.

FIG. 66A shows one embodiment of a modified nucleotide comprising anacyclic structure lacking the (deoxy)ribose furanose ring. FIG. 66Bshows a modified nucleotide comprising the furanose ring. PG1 stands forprotective group that is selectively removable and, and CL stands forcleavable linker, which is also selectively cleavable. Severalparticular embodiments of this invention are contemplated. In oneembodiment these nucleotide compositions can be incorporated into thenucleic acid by nucleic acids modifying enzymes in a controlled fashion,for example to decode the identity of the bases encoded by the markermoiety M. Once the identity of the base has been decoded, then themarker moiety can be cleaved off and removed. The present inventioncontemplates using either or both in the sequencing protocol. Both canhave a variety of different labels and protecting groups (as describedherein).

“Native nucleotide” refers to a nucleotide occurring in nature, such asin the DNA and RNA of cells. In contrast, “modified, nucleotide” refersto a nucleotide that has been modified by man, such as using chemicaland/or molecular biological techniques compared to the nativenucleotide. The terms also include nucleotide analogs attached to one ormore probes to facilitate the determination of the incorporation of thecorresponding nucleotide into the nucleotide sequence. In oneembodiment, nucleotide analogues are synthesized by linking a uniquelabel through a cleavable linker to the nucleotide base or an analogueof the nucleotide base, such as to the 5-position of the pyrimidines (T,C and U) and to the 7-position of the purines (G and A), to use a smallcleavable chemical moiety to cap the 3′-OH group of the deoxyribose orribose to make it nonreactive, and to incorporate the nucleotideanalogues into the growing nucleotide sequence strand as terminators,such as reversible terminators and irreversible terminators. Detectionof the unique label will yield the sequence identity of the nucleotide.Upon removing the label and the 3′-OH capping group, the polymerasereaction will proceed to incorporate the next nucleotide analogue anddetect the next base. Exemplary fluorescent moieties and fluorescentsemiconductor crystals are described in Ju et al., U.S. Pat. No.6,664,079, hereby incorporated by reference.

Other nucleotide analogs that contain markers, particularly cleavablemarkers, are also contemplated, such as those configured using allylgroups, azido groups, and the like, and which are further describedbelow. The nucleotide compositions of the invention are particularlyuseful in massively parallel DNA Sequencing By Synthesis (SBS)approaches utilizing fluorophores as markers.

a. Allyl Analogs

Cleavable fluorescent nucleotides with photo-cleavable linkers havingreversible terminator allyl groups have been described in Ruparel et al.(2005) Proc. Natl. Acad. Sci. 102 (17) 5932-7. Similar, fluorescentnucleotide conjugates have been described in Bi et al. (2006) J. Am.Chem. Soc. 128 (8) 2542-3. In one embodiment, the invention contemplatesusing nucleotide analogs with cleavable markers conveniently configuredwith allyl groups. In a particular embodiment, the exposed amine groupsof incorporated nucleotides are capped during sequencing. In otherembodiments, the nucleotide derivatives comprise two or more allylethers and synthetic intermediates thereto.

Sample preparation and parallel sequencing steps are exemplified, butnot limited, to those illustrated in FIGS. 4 and 5. FIG. 4 Panel A showshow one isolates and prepares the DNA prior to sequencing and Panel Bshows the sequencing cycle. One isolates DNA from a biological sourceand shears it by a mechanical device to the desired average size. Oneend-repairs, A-tails, and circularizes the fragments using a dT-tailedlinker about 100 nucleotides in length. The linker consists of twooutward directed primer recognition sequences and an arbitrary sequenceof about 100 bases between the priming sites. After ligation, onedecomposes noncircular sequences by treatment with an endonuclease. Onedilutes the circular DNA fragments to prepare them for bead-basedemulsion PCR using a biotinylated forward primer and a bead-attachedreverse primer carrying an azido group on its 5′-end. One performsemulsion PCR. An aqueous mix containing all the necessary components forPCR plus primer-bound beads and template DNA are stirred together withan oil/detergent mix to create microemulsions. The aqueous compartmentscontain an average of less than one template molecule and less than onebead. The microemulsions are temperature-cycled as in a conventionalPCR. If a DNA template and bead are present together in a single aqueouscompartment, the bead-bound oligonucleotides act as primers foramplification. One breaks the emulsion and subjects the mixture to anenrichment step by using streptavidin coated magnetic beads. Onedenatures the nucleic acid immobilized on the beads generating singlestranded amplicons to which a self-priming hairpin moiety is thenligated.

The beads are then arrayed on a chip surface and the sequencing bysynthesis reactions are performed. Each cycle comprises steps that areused to read out the DNA sequence (See FIG. 4, Panel B). One subjectsthe array segment to the fluorescent nucleotide conjugate with ahydroxyl-protecting group on the 3′ end. One scans the array and thefluorescent output of each of the fluorescent markers and measures theoutput for each position. One exposes the array to conditions forcleavage of the fluorescent marker and the hydroxyl-protecting group.The entire process is repeated with another set of nucleotide bases unitthe sequence of each position is determined. As the sequence data isgenerated, one collects the sequence information and aligns thereference sequences for diagnosis. One may use computer software and adatabase of previously known mutations and corresponding sequences tocorrelate them to the sequence with known mutations.

The PCR approach described above ensures that instead of sequencing ofthe entire pool of templates, one performs clonal or digital sequencing,resulting in much higher sensitivity for detection of mutations. Forexample, if a spontaneous mutation is present at only 5% of thepopulation and the remaining 95% of the gene copies are wild types it isdifficult to detect the mutated DNA using a conventional pool sequencingapproach because of insufficient sensitivity. In the applicants'approach, one dilutes the input sample so that each PCR emulsion bubblecontains at most a single template, which is then subjected tosequencing. If one performs this process on 1,000 unique clones, thenone on average detects mutant sequences (present in 5% of amplicons) in50 reactions and wild type sequence in 95% of the reactions.

b. Azido Analogs

Nucleotide analogs that contain cleavable markers configured using azidogroups are also useful in the invention's methods and compositions. Thenucleotide analogs are exemplified by

nucleotide compositions of the above general structure (and thestructures in FIG. 66), where PG1 stands for protective group that isselectively removable and, and CL stands for cleavable linker, which isalso selectively cleavable, and R is selected from the group of H, OH,F, NH₂. Several particular embodiments of this invention arecontemplated. In one embodiment these nucleotide compositions can beincorporated into the nucleic acid by nucleic acids modifying enzymes ina controlled fashion to decode the identity of the bases encoded by themarker moiety M. Once the identity of the base has been decoded, thenthe marker moiety can be cleaved off and removed. This inventioncontemplates the use of the cleavable linkers based on the “trimethyllock” mechanism or the “1,6-rearrangement” mechanism. The3′-O-protective groups, which act as reversible terminators, can also becleaved off to enable addition of the next nucleotide. This inventioncontemplates the use of azidomethyl, methylaminoxy, disulfide and allylgroups as reversible 3′-OH terminators.

Methods for synthesizing exemplary nucleotide analogs that containcleavable markers configured using azido groups are described inExamples 2-11 and shown in FIGS. 20-26.

The invention contemplates the use of the cleavable linkers based on the“trimethyl lock” mechanism or the “1,6-rearrangement” mechanism. The3′-O-protective groups which act as reversible terminators can also becleaved off to enable addition of the next nucleotide. The inventioncontemplates the use of azidomethyl, aminooxy, methylaminoxy and allylgroups as reversible 3′-OH terminators.

i. Cleavable Linkers (CI)

Cleavable linkers are exemplified by trimethyl lock based linkers and1,6-rearrangement linkers as further described below.

1. Trimethyl Lock Based Linkers

Cleavable linkers are the linkers linking the marker molecule M to thebase and these can be selectively cleaved using specific cleavingagents. Specifically, this invention contemplates the use of a“trimethyl lock” structure as the cleavage mechanism. These structuresare well known in the chemical arts and have been used before incontrolled drug release applications. The general structures ofcleavable trimethyl lock based linker utilized in particular embodimentsof the present invention are shown below:

The above shows exemplary embodiment A where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from NH, O, S.

The above shows exemplary embodiment B where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is NH.

The above shows exemplary embodiment C where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from NH, O, S, and Y is a selectively removableprotective group.

The above shows exemplary embodiment D where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, X is NH, and Y is anazidomethyl group.

The cleavage mechanism for the trimethyl lock based compounds is shownschematically in FIG. 6. This phenomenon has been previously describedin the chemical literature and used as for basic research studies(Borchardt and Cohen (1972). J. Am. Chem. Soc. 94(26): 9166-9174, Wanget al. (1996) Bioorg. Chem. 24: 39-49), as caging agents for controlleddrug delivery (Wang et al. (1997). J. Org. Chem. 62(5): 1363-1367) andas protective groups in organic synthesis (Wang et al. (1995). J. Org.Chem. 60(3): 539-543).

The linkers in the present invention leverage the ability of thetrimethyl lock system to create cleavably linked nucleotides.

2. 1,6-Rearrangement Linkers

The invention contemplates another category of cleavable linkers linkingthe detectable marker moiety to the nucleotide that are based on 1,6quinone methide rearrangement mechanism (Carl et al. (1981). J. Med.Chem. 24(5):479-480; Duimstra et al. (2005). J. Am. Chem. Soc. 127(37):12847-12855). These structures are well known in the chemical arts andthey have been used before for the controlled drug release applicationsand for chemical synthesis (Azoulay et al. (2006) Bioorganic & MedicinalChemistry Letters 16(12): 3147-3149; Murata et al. (2006) TetrahedronLetters 47(13): 2147-2150). The general structures of cleavable 1,6rearrangement mechanism based linker utilized in some embodiments of thepresent invention are shown below:

The above shows exemplary embodiment E, where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker and Y is a selectivelyremovable protective group.

The above shows exemplary embodiment F, where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker.

The above shows exemplary embodiment G where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from the following: NH, O, S.

FIG. 7 shows an exemplary cleavage mechanism for the cleavable linkerdescribed in the following exemplary embodiment G.

The above shows exemplary embodiment H where BASE is selected from anyribo- or deoxyribo-nucleobases: adenosine, cytidine, guanosine,thymidine and analogs, M is a detectable marker, and X is a divalentgroup selected from the following: NH, O, S. The cleavage is driven hereby the reducing agent and nucleophilic attack of the resulting aminogroup on the carbonyl followed by cyclization. This mechanism has beenused before for the development of protective groups for applications inthe carbohydrate and nucleoside chemistry (Wada et al. (2001).Tetrahedron Letters 42(6): 1069-1072; Xu et al. (2002) CarbohydrateResearch 337(2): 87-91).

The cleavable linker attachment to the base moiety can be achieved invariety of ways that are well known in the art. Among these is the useof linkers based on 1) propargylamino nucleosides, 2) aminoallylnucleosides, 3) propargylhydroxy nucleosides, and 4) hydroxymethylnucleotides as described in co-pending application Ser. No. 61/171,975,hereby incorporated by reference.

A variety of cleavable linkers can be used in this invention and aredescribed in the literature. The linkers may be cleavable chemically,cleavable by light or by other physical means. WO99/57321 describes theuse of nucleotides comprising fluorophores linked to the nucleotide bychemically or photochemically cleavable linker moieties (Olejnik et al.,Proc. Natl. Acad. Sci., 92:7590-7594 (1995). For example, the linkerscould contain disulfide bonds (Mitra, R. D., Shendure, J., Olejnik, J.,Edyta-Krzymanska-Olejnik, and Church, G. M. (2003) Anal Biochem.320(1):55-65) cleavable under reducing conditions (or contain cis-diollinkages cleavable by periodate (WO/53812). The linkers could also becleavable by enzymes (WO 01/92284). Other types of acid or base,oxidative or reducing agent cleavable linkers are also contemplated aswill be recognized by the skilled in the art. Examples of such linkersare described in the literature (Greene et al., Protective Groups InOrganic Synthesis, Second Edition, John Wiley & Sons, (1991). Oneparticularly useful cleavable linker contemplated by this invention isthe linker comprising the substituted azidomethyl ether (WO2004/018493).Another preferred class of cleavable linkers comprisehydrocarbyldithiomethyl groups as described in the U.S. Pat. No.7,279,563, hereby incorporated by reference.

ii. Protective Groups (PG1)

The invention contemplates nucleotide compositions comprising thefollowing protective groups (PG1) that reside on the 3′-OH groups of thenucleotides: 1) 3′-O-Azidomethyl ethers, 2) 3′-O-disulfide, 3)3′-O-methylaminoxy, and 4) 3′-O-allyl.

With respect to the 3′-O-Azidomethyl ethers, exemplary protective groupsthat reside on the 3′-OH groups of the nucleotides that are within thescope of this invention are 3′-O-azidomethyl groups. These groups can beremoved using mild reducing agents, such asTris(2-carboxyethyl)phosphine (TCEP).

With respect to the 3′-O-disulfide group, the 3′-O-disulfide group canbe removed under mild oxidative conditions, for example using in usingmild reducing agents, such as Tris(2-carboxy-ethyl)phosphine (TCEP).

With respect to the 3′-O-methylaminoxy group, the 3′-O-methylaminoxy(3′-O—CH2-NH2) group can be removed under mild oxidative conditions, forexample using in situ generated nitrous acid (such as from sodiumnitrite).

As to the 3′-O-allyl group, this protective group can be removed using avariety of reducing agents, including transition metal complexes (Pd,Rh).

In one embodiment, a 3′-O—CH2-SSSS—R(3′-O-(Alkyl-tetrasulfanyl-methyloxy) group is also contemplated. Thesynthesis of these derivatives has been established and is described inthe art (Tetrahedron, Vol. 61, Issue 4, 2005, 965-970) These protectivegroups can be removed using variety of reducing agents and is cleavedinto the corresponding 3′-OH derivative, for example by usingtris(carboxyethyl)phosphine (TCEP) (Int. J. of Mass Spectrometry, Volume209, Issue 1, 24 Aug. 2001, Pages 47-55).

c. 3′-O-protected nucleosides and nucleotides

The invention contemplates compositions comprising compounds of thefollowing general structure:

PG1 stands for protective group that is selectively removable and, andCL stands for cleavable linker, which is also selectively cleavable. Inone embodiment these nucleotide compositions can be incorporated intothe nucleic acid by nucleic acids modifying enzymes in a controlledfashion for example to decode the identity of the bases encoded by themarker moiety M. Once the identity of the base has been decoded, thenthe marker moiety can be cleaved off and removed. In one embodiment, theinvention contemplates the use of cleavable protection for 3′-OH innucleotides for reversible terminators for SBS.

Examples of PG1 protective groups are shown in FIG. 27. As anillustration, the synthesis of one of the embodiments in such classes ofnucleotide-3′-O—(CH₂ONH₂)-dNTPs is presented in FIG. 28. Briefly, theprotected 3′-methylthiomethyl nucleoside (1) upon treatment with SO₂Cl₂produce activated product (2) which after reaction with hydroxylamine orits N-Fmoc protected compound install aminoxy group. The later compoundscan be triphosphorylated to result in nucleotides. Other compounds andexemplary synthesis pathways within the scope of the invention are shownin FIGS. 29-31.

The nucleotide analogs compositions shown in FIG. 66A comprise theacyclic structure lacking the (deoxy)ribose furanose ring. Thesenucleotide analogs have been described (Gardner, A. F. and Jack, W. E.(2002) Nucl. Acids Res., 30, 605-613) and can be incorporated with highefficiency into the DNA by archeon and Taq DNA polymerases. Theseanalogs compared to regular deoxyribose and ribose analogs lack thefuranose ring and are easier to synthesize. The same Archeonpolymerase's mutants are also known to incorporate a variety ofterminating nucleotides (Gardner, A. F. and Jack, W. E. (1999) Nucl.Acids Res., 27, 2545-2555). In addition this invention contemplates theuse of other suitable nucleotide analogs that have the ability to formthe correct pairing with the nucleic acid molecules and are incorporatedby enzyme or by chemical means. Examples of these include, but are notlimited to 2′,3′-Dideoxy-3′-thionucleotides and their reversibly3′-protected forms (Organic Letters, 2007, Vol. 9, No. 6, 1161-1163),Glycerol Nucleoside Triphosphates (Organic Letters 2006, Vol. 8, No. 23,5345-5347) α-L-Threofuranosyl nucleoside triphosphates (J. Am. Chem.Soc. 2003, 125, 856-857).

D. Reducing Lead and Lag

The cleaving agent is designed to cleave the 3′-OH or the dye attachedto the nucleotide or both the 3′-OH and the dye. A variety ofchemistries may be used for these attachments. FIG. 35 shows variouspossible chemistries for the 3′-OH group. FIG. 36 shows disulfidelinkers for attaching the dye. Importantly, for any particularnucleotide, the chemistries may be same or may be different. Forexample, in one embodiment, the 3′-OH group can carry an azidomethylether and yet the dye can be attached via an azido linker. In anotherembodiment, however, the 3′-OH group can carry an azidomethyl ether andyet the dye can be attached via a disulfide linker. Both the azidomethylether and the disulfide linker are cleavable by TCEP(Tris-carboxyethyl)phosphine, although the disulfide linker cleaves muchfaster than the 3′-O-azidomethyl ether. In one embodiment, two cleavagesteps are contemplated. In the case of disulfide cleavable linker andazidomethyl ether combination, a two-step cleavage could be used and isadvantageous. Lower concentrations of the cleaving reagents can be usedto achieve 2-step cleavage. This has an advantage of being able tobetter wash out the cleaving reagents after the cleavage is completeand, in this way, reduce any potential lead problems. This may alsoresult in lower concentration of cleaving agent scavengers used in theextension steps. The cleaving agent scavengers need to be compatiblewith the polymerase and as such the working concentration may belimited. In the case of disulfide cleavable linker and azidomethyl ethercombination, CLEAVE A could be any reagent that can cleave efficientlythe disulfide bridge. In one embodiment, the use of simple thiols at lowconcentration is contemplated. For example, cysteamine, betamercaptoethanol, dithiothreitiol, reduced glutathione could all be usedat low concentration (1-2 mM) to achieve complete disulfide bridgecleavage. CLEAVE B would comprise, in one embodiment, a reducing agentcapable of reducing azido group and resulting in self elimination andconversion of the aminomethyl ether to the hydroxyl group. This could beachieved by using variety of reducing agents, for example solutions ofboroydrides, phosphines etc. (Chem Rev vol. 88, 297 (1988). One exampleof the cleaving agent is TCEP (Tris-carboxyethyl)phosphine which couldbe used in this case at a lower concentration, e.g., 10 mM. In anotherembodiment, the cleavage of the disulfide bridge is achieved using TCEP(Tris-carboxyethyl)phosphine at 1 mM concentration and at pH 5.5-6.0,while the cleavage of azidomethyl ether is achieved by using TCEP(Tris-carboxyethyl)phosphine at 10 mM an at pH 8-9.

In the case of one step cleavage, the cleaving agent is used atrelatively high concentration (50-100 mM) to achieve fast and efficientcleavage at both locations. It is important for the sequencing processto remove any traces of cleaving agent in the wash steps, as thesetraces could interact with the Extend A and B solution (see thediscussion of these solutions above) in the next cycle and create nativenucleotides. This is highly undesirable as this leads to sequencedephasing (lead and lag) and limits useful read lengths.

One approach might be to increase the number of washes. However, it hasbeen found empirically that increased washing cycles after cleavage stephave only minimal effect on the sequencing performance unless very highnumbers of washes are used (see Example 14). Such an approach would slowdown the process considerably.

The present invention, in one embodiment, contemplates a differentapproach to solving the problem. In one embodiment, the presentinvention contemplates novel compositions to be used in one or more ofthe solutions employed in the sequencing by synthesis method (or in anew, additional separate solution) that reduce, minimize and/or inhibitthe cleaving agent and the “pre-cleaving” effect. In one embodiment, acleavage agent “scavenger” is contemplated. The cleavage agent scavengeris designed to react with any leftover cleaving reagent remaining in theflow cell or the fluidics (e.g. tubing) by inefficient or incompletewashing. In one preferred embodiment, the scavenger agent is added tothe wash solution directly after the cleave step. In another embodimentthe scavenger is added to the Extend A solution. In yet anotherembodiment the scavenger agent is added to Extend B solution. Thescavenger requirements are as follows: 1) solubility; 2) fast andspecific reaction with the cleaving agent. In the embodiments where thescavenger is added to Extend A or B solution, there is the additionalrequirement of lack of inhibition of polymerase reaction and lack ofreactivity with functional groups on the nucleotides, dyes orpolymerase.

In one particular embodiment, the scavenger agent mimics the structureof the protective group present on the 3′-OH location of the nucleotide.In another embodiment, the scavenger mimics only the reactivity of theprotective group. For example, in case of 3′-O-azidomethyl nucleotidesscavenger compounds could comprise azidomethyl, azidoethyl ethers ordisulfide compounds. In case of 3′-O—NH2 nucleotides the scavengerscould be any aminoxy compounds, such as hydroxylamine. In case of3′-O-allyl nucleotides the scavengers could be any allyl ether ordisulfide compounds. FIG. 37 provides examples of cleaving agent“scavengers.” It has been found empirically (see Example 15), that theuse of such compounds improves base calling accuracy, without the needfor additional wash steps (and in particular, without the need for highnumbers of wash cycles).

E. Dephasing

Many next-generation DNA sequencing systems read the sequence ofmillions of different single-stranded DNA fragments in parallel by usinga polymerase enzyme to incorporate fluorescently labeled DNA nucleotidesinto the complementary strand one cycle at a time. However,incorporation errors can shift the phase of some of the templates, sobase read outs may lead ahead or lag behind the cycle number. Theinvention provides a model and methods to account for incorporationerrors and show how the model may be inverted to correct this dephasingand extend read lengths.

Although fluorescence-based, single-molecule sequencing on a chip hasbeen demonstrated, it is very sensitive to polymerase incorporationerrors. This may be reduced and therefore reliability of sequence readout may be increased if each spot on a chip is an ensemble of identicaltemplate molecules. Polymerase errors (such as the incorporation of thewrong complementary nucleotide or no incorporation at all) areinevitable, but infrequent. Therefore, the superposition of all of thefluorescent signals from template molecules within an ensemble willprimarily be from the correct nucleotide. As the number of cycles getslarge, however, certain errors can accumulate within an ensemble andcontribute to possible miscalling of the correct nucleotide.

For our analysis, we assume that a set of reversibly terminated andcleavably labeled nucleotides with four different dye colors (one foreach nucleotide type: A, C, G and T) are used for sequence read out. Themethods described herein may also be applied to other types of SBSprocesses such as pyrosequencing. If the SBS process works withoutmis-incorporations, then for each cycle only a single nucleotide type isincorporated into every strand in an ensemble. During a read out phase,the color of each ensemble is measured, then during a cleavage phase,the terminator and dyes are cleaved off and the chip is ready for thenext cycle. Thus, the position of the base being read out on everytemplate on the chip is synchronized with the cycle number.

Because of impurities, limited polymerase efficiencies and other errors,some of the templates within an ensemble may get out of phase with thecycle number. For example, the base that is incorporated in the i^(th)cycle may be complementary to the i−1^(st) position or the i+1^(st)position in the template rather than the expected i^(th) position. Theinvention's methods provide computational re-phasing of the dephaseddata. FIG. 8 is a schematic flow chart for re-phasing. Additional datademonstrating the efficacy of the invention's methods is discussedbelow, including FIGS. 9-19 and Example 11 (FIGS. 33-34).

a. Sequence Lead

Polymerases that have an increased capacity for incorporating 3′reversibly terminated nucleotide analogs continue to have a preferencefor incorporating native nucleotides. This means that even thoughnucleotide analogs may be extremely pure, any residual nucleotides with3′-OH (non-terminated) will be incorporated at a much higher rate andtherefore appear to be more prevalent. The incorporation ofnon-terminated nucleotides has the effect of skipping a base, as asecond incorporation (the next base) will occur in the same cycle. Thus,the fluorescent measurement for that template will exhibit the dye fromthe following base rather than the expected base at that cycle number.Since that template now exhibits a “lead,” it will continue to do so,even if all future nucleotides are reversibly terminated. This effect iscumulative and shown in simulated data in FIG. 9 for a non-terminatednucleotide incorporation rate of 2% as compared to the terminatednucleotide analog rate and a repeated 35 base sequence of ACTGACTGACTG .. . . Here we make the assumption that each of the nucleotides has thesame nonterminated incorporation rate thereby allowing us to use alinear model. Again, the actual nucleotide purity may well be betterthan say 99.5%, but the apparent non-terminated incorporation rate maybe 2% depending on the polymerase. In the example in FIG. 9, the modeltells us the amount of signal due to the sequence lead effect. In cycle20, the model calculates that we have 60% of the signal from the primarybase at the 20th position (red), 32.4% of the signal from the base atthe 21st position (blue), 6.7% of the signal from the base at the 22ndposition (green), 0.8% of the signal from the base at the 23rd position(yellow), and 0.07% of the signal from the base at the 24th position(red). An interesting observation is that with the lead effect, theprimary base signal (actual base at that cycle) does not have a 100%signal as some templates are already “reading out” subsequent bases onthat strand. Thus at the end of a run, we can “look forward” and shiftback the lead signals and correct the primary signals. We denote thecontributions at the i^(th) cycle as R_(0Lead,i), R_(+1Lead,i),R_(+2Lead,i), R_(+3Lead,i), etc. for the ratio between the reducedsignal for the i^(th) base to the actual i^(th) base population, theratio contribution to the i^(th) base signal from the i+1^(st) base, theratio contribution to the i^(th) base signal from the i+2^(nd) base,etc. Because the amount of lead changes with each cycle, there will be adifferent set of ratios for each cycle.

b. Sequence Lag

We developed a model for de-phasing due to sequence lag. This is causedby limited incorporation efficiency where some small percentage of thetemplates do not get a base incorporated in the cycle. FIG. 10 showssimulated data for a 98% incorporation efficiency for the same templatesequence as in FIG. 9. We denote the contributions at the i^(th) cycleas R_(0Lag,i), R_(−1Lag,i), R_(−3Lag,i), etc., for the ratio between thereduced signal for the i^(th) base to the actual i^(th) base population,the ratio contribution to the i^(th) base signal from the base, theratio contribution to the i^(th) base signal from the i−2^(nd) base,etc.

c. Nucleotide Incorporation Events

As discussed above, every time there is an available site for thepolymerase to incorporate a nucleotide on a template, there are threepossible outcomes: First, no nucleotide is incorporated—Event No-I. Ifno nucleotide is incorporated due for example to polymeraseinefficiency, then the site remains available for the next cycle. Weterm this a “lag” event as it has the effect of causing a readout in thenext cycle that will be from the position behind or lagging the cyclenumber. Second, a reversibly terminated nucleotide is incorporated—EventT-I. If as expected, a reversibly terminated nucleotide is incorporated,then the nucleotide readout is in synchrony with the cycle number. Inthe next cycle, the next consecutive template nucleotide position willbe available for incorporation. Third, a non-terminated (native)nucleotide is incorporated—Event N-I. If a non-terminated nucleotide isincorporated, then during that same cycle, there is a second opportunityfor another nucleotide to be incorporated at the subsequent position inthe template strand. We term this a “lead” event as it has the effect ofcausing a readout of a nucleotide that is at a position that is ahead ofor leading the cycle number. This second incorporation event is subjectto the same three possible outcomes (No-I, T-I or N-I); thus, N-I eventsare recursive.

We will use the variable G_(i) to represent the rate at which a lagoccurs at template position i and similarly D_(i) for the lead rate atposition i. The analysis assumes that these rates may vary from positionto position depending on the identity of the nucleotide that is to beincorporated, but we have assumed that all incorporation events for aparticular nucleotide have the same lag and lead rates, even if theincorporation is not the first one in a cycle (it follows an N-I event).The fluorescent signal that will be generated from an incorporationevent at a template location i is proportional to (1−G_(i)−D_(i)), so atevery i^(th) incorporation event, the three types of events (No-I, T-Iand N-I) will occur at the following rates: Event No-I at rate G, EventT-I at rate (1−G_(i)−−D_(i)), and Event N-I at rate D_(i).

d. Signals Produced in Each Cycle

Although there are only three potential outcomes from an incorporationevent, all of the combined events from multiple cycles in a template canbe fairly complex. FIG. 11 may be used to better visualize the sequenceof extension events. For simplicity, only 4 templates positions and only3 cycles are illustrated in FIG. 11. The numbered regions in thevertical direction along the left edge indicate the nucleotide positionalong the strand. The horizontal direction symbolizes the relativenumber of strands in an ensemble that undergo events No-I, T-I or N-I(lag, readout or lead). The various events for cycle 1 in the sequenceprocess are shown in shades of blue, events for cycle 2 are shown in redand events in cycle 3 are shown in green.

For clarity in FIG. 11, we have designated each of the three possibleevents (No-I, T-I and N-I) to occur at the same rate for every cycle. Inan actual system, the lead and lag rate are both likely to be muchsmaller values. The chart is easier to understand if it is viewed onecolor at a time. The blue regions represent events that occur during thefirst cycle. At position 1 of the template, the entire ensemble oftemplates are available for extensions, thus (light blue) undergo a lag(no incorporation), (medium blue) are read out and (dark blue) undergo alead. The portion of templates that experienced a lag (light blue)during the first cycle, remain available during the second cycle forincorporations. The portion of the templates that experienced a readout(medium blue) comprises the signal that is read during cycle 1 atposition 1. This portion will progress in synchrony and allowincorporations to occur at position 2 during the second cycle. Theportion of the templates that experienced a lead at position 1 will havea second incorporation event during cycle 1 at the second position ofthe template. This incorporation again will be split equally into thethree possible events. A portion of the templates will remain unextended(lag), a portion will generate a signal (readout) and a portion willundergo a lead and produce a third set of incorporation events atposition 3. This process will continue during cycle 1. Although leadevents may continue down the entire length of the template during cycle1, in practical terms, the effects after about 4 lead events arenegligible.

In cycle 2 (red colors), the only strands that are available to beextended at position 2 are those for which one of three events occurred(see FIG. 12): (1) strands that were read out at position 1 during cycle1, (2) strands that experienced both a lead at position 1 and a lag atposition 2 during cycle 1, and (3) strands that experienced a lag atposition 1 during cycle 1 along with a subsequent lead at position 1during cycle 2. A portion of these strands will also experience a cycle2 lead to the third position, however, since they have “caught up” tothe other strands with available sites at the second position, they arelumped together with them and further leads are considered as portionsof the combined population.

Similar events occur at each template position during cycle 2. Theevents of cycle 3 (green shades) follow very similar patterns to thosedescribed for cycle 2.

e. Mathematical Models of Dephasing and Rephasing

We may derive general equations that describe all the incorporationevents at each position and for each cycle. If we denote the relativemagnitude (out of 1) of the number of strands that remain unincorporatedfor position i at the end of a cycle j as R_(i,j), and the number ofstrands that are available for incorporation in the next cycle asA_(i,j) then

R _(i,j) =R _(i,j-1) −A _(i,j-1)(1−G _(i))  (1)

and

A _(i,j) =R _(i,j) −R _(i-1,j) +A _(i-1,j) D _(i-1)  (2)

To explain the derivation of Equation 1, we use the example in FIG. 13,which shows a portion of the chart from FIG. 11. Only cycles 2 and 3 areshown for position 2. R_(2,3) is comprised of R_(2,2) minus a portion(1−G₂) of A_(2,2).

To explain the derivation of Equation 2, we use the example in FIG. 14,which shows a portion of the chart from FIG. 11. Only cycle 3 is shownfor positions 2 and 3. A_(3,3) is comprised of R_(3,3) minus R_(2,3)plus the lead portion (D₂) of A_(2,3).

It should be noted that for any particular cycle and position, thenumber of available strands, A_(i,j) is generally fewer than the numberof remaining strands, R_(i,j), since some templates at the particularposition are still lagging and unavailable, but may “catch up” in futurecycles.

The signal that is produced, S_(i,j), at the i^(th) position at the endof the j^(th) cycle comes from the proportion of the strands that areavailable, A_(i,j), that undergo event T-I

S _(i,j) =A _(i,j)(1−D _(i) −G _(i)  (3).

In one embodiment, the model is used to apply the lead-lag compensationbased on calibration of parameters, before or during the test, and toprovide an initial estimate of the base identity at each location asdetermined during the sequencing run. In a particular embodiment, G_(i)and D_(i) for each nucleotide may be pre-calibrated or measured duringthe sequencing procedure. In general, the model is constructed with lagparameters that are applied to each cycle and lead parameters that arerecursively applied to each cycle.

In a particular embodiment, the lead-lag matrix is formulated after aninitial draft sequence is measured. This allows application of theproper set of G_(i) and D_(i) parameters to each cycle based on thenucleotides identified at each position in the draft sequence. In afurther embodiment, the re-phasing of data is iterated using the resultof one re-phasing calculation to select an updated set of G_(i) andD_(i) parameters for the next iteration.

f. Simulated Dephased Sequencing Data

We may use the relationships derived in the previous section to generatesequence data that simulate the signals that might occur when portionsof every incorporation undergo a lead and a lag. As an example, we usethe lead and lag factors below and generate simulated sequence dataassuming the maximum signal from the template is 10,000 counts and thetemplate has a 35 base repeating sequence of AGCTAGCTAGCT. FIG. 15 showsdata generated from the iterative application of equations 1-3 using theparameters in Table 1.

TABLE 1 Lead Factors and Lag Factors for Nucleotides A G C T LeadFactors 4.10% 4.20% 4.30% 4.40% Lag Factors 1.10% 1.20% 1.30% 1.40%

FIG. 15 shows that with the presence of a lead and lag component, thereis a cumulative effect that reduces the signal from the expectednucleotide at a particular cycle and “spreads” some of the signalforwards and backwards. As the number of cycles increases, it becomesmore and more difficult to directly read the correct base from thegraph, thereby limiting the effective read length of the template.

g. Re-Phasing Sequencing Data

Data herein (Example 11, FIGS. 33-34) demonstrate that applying themethods and the below described equations of the invention, exemplary16-base and 25-base nucleotide sequences were sequenced with highfidelity. The high quality of the data, particularly in the last severalbases in FIG. 34, demonstrates that the read length will not be limitedby signal decline. Thus, it is contemplated that the invention's methodsare applicable to sequences containing at least 16 nucleotides, at least25 nucleotides, at least 35 nucleotides, at least 50 nucleotides, atleast 100 nucleotides, at least 1,000 nucleotides, and at least 10,000nucleotides. Further description of the equations used to re-phasesequencing data is described as follows.

We constructed a matrix equation that describes a model for the reducedmeasured signal, I_(Mi), from the lead and lag effect in the i^(th)cycle from the original template populations, I_(Ai), for all cycles,i=1 to N. Here each of the intensity matrices ([I_(Ai)] and [I_(Mi)])have N rows (one for each cycle) and four columns (one for each color).

$\begin{matrix}{{\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix} = {K_{{Lead}\text{/}{Lag}}\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix}}},} & (4)\end{matrix}$

where the lead/lag matrix, K_(Lead/Lag), is a square N×N matrix of thefollowing form:

$\begin{matrix}{K_{{Lead}\text{/}{Lag}} = \begin{bmatrix}R_{{{Lag}\text{/}{Lead}},1} & R_{{{+ 1}{Lead}},1} & R_{{{+ 2}{Lead}},1} & R_{{{+ 3}{Lead}},1} & \ldots & R_{{{+ {({N - 1})}}{Lead}},1} \\R_{{{- 1}{Lag}},2} & R_{{{Lag}\text{/}{Lead}},2} & R_{{{+ 1}{Lead}},2} & R_{{{+ 2}{Lead}},2} & \ldots & R_{{{+ {({N - 2})}}{Lead}},2} \\R_{{{- 2}{Lag}},3} & R_{{{- 1}{Lag}},3} & R_{{{Lag}\text{/}{Lead}},3} & R_{{{+ 1}{Lead}},3} & \ldots & R_{{{+ {({N - 3})}}{Lead}},3} \\R_{{{- 3}{Lag}},4} & R_{{{- 2}{Lag}},4} & R_{{{- 1}{Lag}},4} & R_{{{Lag}\text{/}{Lead}},4} & \ldots & R_{{{+ {({N - 4})}}{Lead}},4} \\\vdots & \vdots & \vdots & \vdots & \ddots & \vdots \\R_{{{- {({N - 1})}}{Lag}},N} & R_{{{- {({N - 2})}}{Lag}},N} & R_{{{- {({N - 3})}}{Lag}},N} & R_{{{- {({N - 4})}}{Lag}},N} & \ldots & R_{{{Lag}\text{/}{Lead}},N}\end{bmatrix}} & (5)\end{matrix}$

The diagonal terms, R_(Lag/Lead,i) in the K_(Lead/Lag) matrix above isthe fractional remaining signal in the i^(th) cycle from the i^(th)position of the templates after all of the leads and lags to that point.Each of the terms in the upper triangular portion of the matrix,R_(+kLead,i), is the fractional contribution to the signal in the i^(th)cycle from k positions forward of the i^(th) position. Each of the termsin the lower triangular portion of the matrix, R_(−kLag,i), is thefractional contribution to the signal in the i^(th) cycle from kpositions before the i^(th) position. In most systems, the terms with kgreater than about 4 (5 positions or more away from the positioncorresponding to the cycle number) are negligible. The diagonal termsare close to 1 for the earlier cycles and do not drop below about 0.25for the later cycles. Thus, this matrix is invertable.

In order to compensate for both sequence leads and lags, we solved for[I_(Ai)] in Equation 4 by taking the inverse, K_(Lead/Lag) ⁻¹, ofK_(Lead/Lag) (Equation 5) to get an estimate of the actual templatepopulation [I_(Ai)]:

$\begin{matrix}{\begin{bmatrix}I_{A\; 1} \\I_{A\; 2} \\\vdots \\I_{A\; N}\end{bmatrix} = {{K_{{Lead}\text{/}{Lag}}^{- 1}\begin{bmatrix}I_{M\; 1} \\I_{M\; 2} \\\vdots \\I_{M\; N}\end{bmatrix}}.}} & (6)\end{matrix}$

When the lead rates for all the nucleotides are identical and the lagrates for all of the nucleotides are identical, then the lead/lagmatrix, K_(Lead/Lag) does not depend on the sequence. This makesEquation 6 linear and the inverse of the matrix is deterministic. Inthis case the inverse of the lead/lag model gives the correction matrix,K_(Lead/Lag) ⁻¹, which is applied just once at the end of a run andtakes into account all of the signals from the first to the last base.

If on the other hand the lead and lag factors vary from one nucleotideto the next, then the lead/lag matrix, K_(Lead/Lag), depends on theactual sequence (solution of [I_(Ai)] in Equation 6) and the problem isnon linear. In other words, we need to determine an estimate for thetrue value of the intensities of each base in the sequence when thegoverning equations depend on this solution.

To solve the non-linear problem, one can estimate a solution and iterateuntil the solutions converge. We may use the raw out-of-phase sequencedata to make an initial estimate of the sequence by taking the maximumvalue at each cycle, using this information to determine the lead andlag rates for each position, construct a lead/lag matrix (K_(Lead/Lag)),take the inverse of that matrix and solve for the corrected, re-phasedsequence. We can then use the new sequence to make a new estimate of thelead/lag matrix, etc. As long as the various lead and lag factors arefairly close to one another, this method should converge in about two orthree iterations.

h. Additional Factors

The above method is a very powerful way of “cleaning up” sequence datathat has been dephased due to the lead and lag phenomena. The matrixcondition number determines when matrix manipulations will be sensitiveto small numerical variations. When the logarithm base 10 of the matrixcondition is less than the number of significant digits of the datacomprising the matrix, then the matrix is well behaved. A largecondition number (where its logarithm base 10 is greater than the numberof significant digits of the data) means the matrix is ill-conditionedor sensitive to the numerical inaccuracies inherent in the data. A goodassumption for the type of data that is generated by fluorescent-basedsequencing systems is that there are less than two significant digits inthe data and the matrix condition number should therefore be less thanabout 20 in order to avoid ill-conditioned matrices.

FIG. 16A shows a simulated 35 base read data with 10% noise added andlead and lag factors of 1% each. FIG. 16B shows an accurate lead/lagcompensation reconstruction using the inverse lead/lag matrix, K⁻¹_(Lead/Lag). The condition number for this lead/lag matrix is 5. FIG.17A shows the same exemplary 35 base sequence with 10% noise and a leadand lag factor of 1.75% each and FIG. 17B shows the reconstruction.Here, the condition number is 550 and reconstruction is poor. FIG. 18plots the matrix condition numbers below 20 (where matrices will bewell-conditioned) for 35 base lead/lag matrices with various values oflead and lag. Because the lead/lag matrix is calculated independent ofthe DNA sequence in a template for the case where nucleotides all haveequal lead ratios and equal lag ratios, we are able to determine ourability to accurately call bases without consideration of the A, C, T,and G content of the templates.

Even if a 35 base lead/lag matrix is ill-conditioned and produces poorreconstruction, smaller matrices from a portion of the same data maystill be well behaved. For example, FIG. 17C shows the reconstruction ofthe first 18 bases for a lead and lag of 1.75% each (same conditionsthat produced the poor reconstruction for the 35 base read). Here the18×18 lead/lag matrix has a condition number of 3.7. The matrix becomesill-conditioning when cumulative contributions from the lead or lag orboth generate signals that are on the order of the signal from the truebase (where the position equals the cycle number).

The above shows that changing the read length can provide an accuratereconstruction of earlier portions of the data. Thus, we can plot thelead and lag factors that will cause matrices of different sizes (readlengths) to have a condition number of 20 (the point where matricesbecome too ill-conditioned for precise reconstruction). For example, ifa 23×23 matrix produces a condition number of 20, then we would restrictthe read length to a maximum of 23 as reconstruction using the 24th base(as well as any additional bases) would likely create a matrix that istoo ill-conditioned to accurately reconstruct the data. FIG. 19 showsthe read lengths for various lead and lag factors. This plot provides amethod for predicting the read length obtainable from a sequencingsystem based on two factors: the purity of the nucleotides (majorcontributor to the lead) and the polymerase incorporation efficiency(major contributor to the lag). This result also shows that if both thelead and lag factors are below about 0.5%, this results inreconstruction of a 100-base read.

Another way of producing more precise reconstructions when the matrixcondition number is too large, is to approximate the lead/lag matrix bythe product of two or more matrices with smaller lead and lag values.For example, the K_(Lead/Lag) ⁻¹ of Equation 6 may be approximated bythe product of two identical matrices, K_(0.5 Lead/0.5 Lag) ⁻¹ eachcorresponding to leads and lags having half the values of the actuallyleads and lags.

K ⁻¹ _(Lead/Lag) =K ⁻¹ _(0.5Lead/0.5Lag) K ⁻¹ _(0.5Lead/0.5Lag)  (7).

FIG. 17D shows the same reconstruction as FIG. 17B, but with Equation 7being used for the value of K_(Lead/Lag) ⁻¹ of Equation 6.Reconstruction using multiple smaller-lead/lag-value matrices givesnearly identical reconstruction of well-conditioned systems as thenominal K_(Lead/Lag) ⁻¹ matrix. For certain ill-conditioned systems, themethod may produce stable reconstructions where the nominal K_(Lead/Lag)⁻¹ matrix will not; however, they are likely to be less precise thanreconstructions in well-conditioned systems.

Although this example describes the matrix approximation using theproduct of two identical matrices with half the lead and lag values, anynumber of matrices in the product and any combination of lead lag valuesmay be used as long as the values produce well-conditioned matrices.

F. Field Flattening

In one embodiment, when a chip of uniform dye concentration is imaged,it may be desirable that all of the pixels in the resultant image havenearly the same intensity, with variations reflecting only therelatively small distribution inherent in the camera's optical system.In practice, however, the inventors have found that variations inillumination and filter response produce a significant spatiallycorrelated pattern in an image. The inventors have also found that thepattern is highly reproducible and has a linear response to changes indye concentration and camera exposure. These conditions lead to thefollowing algorithm for removing this spatial correlation between pixelintensity and location of the pixel on the solid substrate.

First, for each machine and each filter, we image the pattern of aspatially uniform fluorescence on a dyed microscope slide. Second, theimage is smoothed using a low-pass filter. In the resultant smoothedimage, M, we choose an origin point, M_(x0,y0). The choice of the pointis fairly arbitrary but it is selected from a region in which thesmoothed images of all of the filters have low intensity gradients tominimize the impact of changes in the system. Third, the intensity ofeach pixel at a point in a raw image (R_(x,y)) is replaced by itsfield-flattening value, F_(x,y), where F_(x,y)=R_(x,y) M_(x0,y0)/M_(x,y)and M_(x,y) is the value of the model image at the same spatial locationas the raw image pixel. The resultant “Field Flattened” image, F, hasintensities that are now solely dependent on the camera exposure and dyeconcentration, and do not have any correlation with the spatial locationof the pixel in the image.

The invention's algorithms and equations for field flattening aredistinguished from those described by, for example Eltoukhy et al.(2006), since the algorithms of Eltoukhy et al. relate to signal noisethat is uncorrelated to system parameters (e.g., uneven light source).In contrast, the signal noise that is corrected by the invention'smethods is correlated to the signal's position across the solidsubstrate. In one embodiment, each pixel is corrected (i.e., fieldflattened) based on a previously calibrated baseline intensity at thatpixel position and a scaling factor based on for example a longerexposure time.

G. Spot Location in the Array

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) forlocating the spots in the array. In one embodiment, the spot locatingimage processing algorithm uses the fact that the spots on the chip arein a regular hexagonal pattern along vertical columns and diagonal rows.To find the columns of spots, image pixel values are summed along thevertical direction. This results in a one-dimensional set of data thatresembles a sinusoidal pattern. The peaks of the pattern are measuredand used to determine the period and phase of the pattern. There arethen used to guide a search to determine a set of equations for verticallines that approximately bisect each of the spots in a column. Theresult is a set of equations for parallel lines (slope, intercept andspacing) at regular spacing. A number of these lines are then probed toestablish a second sinusoidal-like pattern of intensities along thelines. These are then used to determine the period, angle and phase ofthe diagonal lines that bisect each of the spots. These second set oflines are at a 60 degree angle from the vertical lines. The intersectionof the diagonal set of lines and the vertical set of lines give anestimate for the subpixel locations of each of the spot centers.

H. Image Sharpening

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) tosharpen the image of the spots on the array. This is particularly usefulif chips are constructed with tightly packed spots. In such a case, itmight be beneficial to run the images through a sharpening filter inorder to reduce the amount of blur or spread for each of the spots. Thiswill reduce the amount of light energy blooming into adjacent spots.Similarly, if the optics for the system cannot sufficiently resolvespots on the chip, then the application of a sharpening filter may alsohelp to precisely analyze the images. A number of sharpening algorithmsmay be used to narrow the spread of the spots. One embodiment uses aWiener filter (as described, for example, in The Image ProcessingHandbook, by John C. Russ, Published by CRC Press, 2006, ISBN0849372542, 9780849372544, 817 pages) to make the diameter of the spotssmaller and remove light energy from adjacent spots.

I. Spot Brightness Determination

In one embodiment, the present invention contemplates a processing step(preferably in a series of processing steps as discussed above) todetermine spot brightness. In one embodiment, the pixels that surroundeach of the subpixel locations of the spots are summed to determine anestimate for the spot brightness. The local set of pixels that isselected depends on both the diameter of the spots and the locationwithin the pixel of the subpixel center location. For example, if thesubpixel location is close to the top of the pixel that contains thecenter, then more pixels above the pixel that contains the center arecounted than pixels below the pixel containing the center.

In one embodiment, the above method for making spot brightnessmeasurements is repeated independently for each of the four differentcolor channels (four separate images) and the sharpening and neighborinfluence (see below) correction calculations are applied, then thecolor crosstalk correction is applied (see below). In one embodiment,the result of the color crosstalk calculation produces a list of fourvalues (one for each dye color) for each spot in the images that may beused to call the base for that sequencing cycle.

J. Neighbor Influence Elimination

In addition to each of the spots expanding beyond its physical bounds,the light from one spot (bead) may illuminate an adjacent spot and makeit appear to have more of the color of its neighbors. This might happenbecause light being emitted from one bead make impinge on an adjacentbead, be reflected within the bead and then reemitted from that bead.This neighbor influence may be eliminated by, in one embodiment,constructing an influence or “spreading” matrix, then applying theinverse of this matrix to the data. To formulate the solution to theneighbor influence problem from spot data that is in hexagonal form, itis convenient to first put the data into a rectilinear array. This isdone by shifting the even vertical columns up by ½ of a pixel as shownbelow. Thus, a two-dimensional rectilinear matrix, whose elementsrepresent the magnitude of each spot in the original image of thehexagonal array of spots, may be used. To further facilitate matrixmanipulations, the rectilinear spot matrix may be made into a spotvector by stacking the columns from the two-dimensional matrix under oneanother to form a one dimensional array or vector. In other words, thesecond column is appended to the bottom of the first, the third to thebottom of the second, etc., thereby generating a 1×N² vector formed froman N×N spot matrix.

In one embodiment, a “spreading matrix” is next formed that representsthe magnitude of the influence from a spot to neighboring spots. In ageneral formulation, a central spot may be thought of as influencing thenearest six neighbors surrounding the central spot by a fraction, A, ofthe central spot brightness, the next nearest neighbors by a smallerfraction, B, etc. If the central spot is very bright, then its neighborsmay appear to be emitting their own light of the same color as thecentral spot, even if they actually generate none of their own light inthat color. The spreading matrix is formulated such that if it isapplied to an ideal image of single element spots (each spot is anidealized point and does not extend beyond one element of the matrix),then the resultant matrix will have spots that have been spread acrossseveral elements due to the neighbor influence phenomenon. Thus, thespreading matrix is a model for the influence of any spot in the imageto any other spot in the image.

For hexagonal arrayed spots that have been make into a one-dimensionalvector with dimensions N²×1 (in other words, a concatenation of all thecolumns of the matrix), the spreading matrix, S, may be formulated as aN²×N² matrix. An example 25×25 spreading matrix corresponding to a 5×5spot image that has the three levels of neighbor influence (A for theclosest 6 neighbors, B for the next closest 6 and C for the thirdclosest 6) is shown below.

1 A C 0 0 A A B 0 0 B C 0 0 0 0 0 0 0 0 0 0 0 0 0 A 1 A C 0 B A A B 0 CB C 0 0 0 0 0 0 0 0 0 0 0 0 C A 1 A C 0 B A A B 0 C B C 0 0 0 0 0 0 0 00 0 0 0 C A 1 A 0 0 B A A 0 0 C B C 0 0 0 0 0 0 0 0 0 0 0 0 C A 1 0 0 0B A 0 0 0 C B 0 0 0 0 0 0 0 0 0 0 A B 0 0 0 1 A C 0 0 A B 0 0 0 B C 0 00 0 0 0 0 0 A A B 0 0 A 1 A C 0 A A B 0 0 C B C 0 0 0 0 0 0 0 B A A B 0C A 1 A C B A A B 0 0 C B C 0 0 0 0 0 0 0 B A A B 0 C A 1 A 0 B A A B 00 C B C 0 0 0 0 0 0 0 B A A 0 0 C A 1 0 0 B A A 0 0 0 C B 0 0 0 0 0 B C0 0 0 A A B 0 0 1 A C 0 0 A A B 0 0 B C 0 0 0 C B C 0 0 B A A B 0 A 1 AC 0 B A A B 0 C B C 0 0 0 C B C 0 0 B A A B C A 1 A C 0 B A A B 0 C B C0 0 0 C B C 0 0 B A A 0 C A 1 A 0 0 B A A 0 0 C B C 0 0 0 C B 0 0 0 B A0 0 C A 1 0 0 0 B A 0 0 0 C B 0 0 0 0 0 B C 0 0 0 A B 0 0 0 1 A C 0 0 AB 0 0 0 0 0 0 0 0 C B C 0 0 A A B 0 0 A 1 A C 0 A A B 0 0 0 0 0 0 0 0 CB C 0 B A A B 0 C A 1 A C B A A B 0 0 0 0 0 0 0 0 C B C 0 B A A B 0 C A1 A 0 B A A B 0 0 0 0 0 0 0 0 C B 0 0 B A A 0 0 C A 1 0 0 B A A 0 0 0 00 0 0 0 0 0 B C 0 0 0 A A B 0 0 1 A C 0 0 0 0 0 0 0 0 0 0 0 0 C B C 0 0B A A B 0 A 1 A C 0 0 0 0 0 0 0 0 0 0 0 0 C B C 0 0 B A A B C A 1 A C 00 0 0 0 0 0 0 0 0 0 0 C B C 0 0 B A A 0 C A 1 A 0 0 0 0 0 0 0 0 0 0 0 00 C B 0 0 0 B A 0 0 C A 1If the spreading matrix, S, is inverted, S⁻¹, it may be used toeliminate the neighbor influence modeled by the spreading matrix. If wemultiply the measured spot matrix (in the form of a vector), v_(spot),by the inverse of the spreading matrix, S⁻¹, we can generate an estimatefor the spot matrix with the neighbor influence removed,v_(uninfluenced)

v_(uninfluenced)=S⁻¹v_(spot).

The method described above for removing the neighbor influence cangenerate sizable spreading matrices on the order of N⁴ and therefore maybe computationally intensive for typical images. Since the influence ofspots that are relatively far from the spot of interest have relativelynegligible influence, it is possible to reduce the size of the spreadingmatrices used for the calculation and perform the calculation on smallersubsections of the image at a time. This can significantly reduce thecomputational complexity and computer memory requirements for thecalculation. It should be understood that the methods set forth abovemay be generalized in algorithms that are more efficient or operate onsmaller portions of the image.

K. Spectral Crosstalk Calibration

In one embodiment, it may be desirable to correct the data to accountfor color crosstalk. This may be done using methods known in the art(e.g., U.S. Pat. No. 7,209,836 incorporated by reference) as well asmethods disclosed herein (see Example 10, FIG. 32). For example, afour-color fluorescent detection system (for detection of the exemplarycolors blue, green, yellow and red) has one detector channel for each ofthe four different color dyes. However, because the dyes have fairlybroad spectra, there is some detection of dyes in adjacent colorchannels. For example, when exciting only a green dye, a signal isvisible in the yellow channel as well as the green channel. If thisspectral crosstalk is calibrated, it may be removed from subsequentmeasurements even if the dyes are mixed in unknown quantities. Todetermine actual fluorescent intensities for the four colors, A, B, Cand D from measured detector outputs, M_(A), M_(B), M_(C), M_(D) incorresponding channels, one needs to know all of the spectral crosstalkfactors: R_(AB), R_(BA), R_(BC), R_(CB), R_(CD), and R_(DC). Forexample, R_(AB) is the ratio between the portion of the signal in the Achannel coming from the B dye and the actual intensity of the B dye. Iffor instance R_(AB) is 20%, then the A channel will have an additionalsignal equal to 0.2 times the actual B dye intensity in the B channel.Thus for channel B, the observed measurement, M_(B), is the directmeasurement of B and the two contributions from the adjacent channels(if any): M_(B)=B+R_(BA)A+R_(Bc)C (6). For the four channels, this maybe written in matrix form:

$\begin{matrix}\begin{matrix}{\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix} = {K\begin{bmatrix}A \\B \\C \\D\end{bmatrix}}} \\{where} \\{K = {\begin{bmatrix}1 & R_{AB} & 0 & 0 \\R_{BA} & 1 & R_{BC} & 0 \\0 & R_{CB} & 1 & R_{CD} \\0 & 0 & R_{DC} & 1\end{bmatrix}.}}\end{matrix} & (7)\end{matrix}$

Each of the six spectral crosstalk factors may be determined through anexperiment with pure dyes. We want to solve for the actual fluorescentsignals, A, B, C and D given the detector measurements, M_(A), M_(B),M_(C), M_(D). Thus, we want to solve the above matrix Equation (7). Thisis equation (8):

$\begin{bmatrix}A \\B \\C \\D\end{bmatrix} = {K^{- 1}\begin{bmatrix}M_{A} \\M_{B} \\M_{C} \\M_{D}\end{bmatrix}}$

where K⁻¹ is the inverse of matrix K. Although the inverse of matrix Kmay be written out in terms of the six spectral crosstalk factors, it issomewhat complex and is best performed by plugging in the numbers andletting the computer take the inverse. The results are discussed inExample 10 which demonstrate that a base in the sequence would have beenmiscalled were the spectral crosstalk calibration not performed.

Any multicolor sequencing by synthesis device may be calibrated usingthe above equations and the resultant spectral crosstalk matrix may beused in all four color measurements from the device. In one embodiment,if we also include information on the relative magnitude of each of thefour colors, then we can also correct for differences in perceived dyebrightness from one channel to the next. Multiplying the matrix K by adiagonal matrix, whose diagonal terms are the relative brightness foreach dye, produces a new matrix K whose inverse will automatically scalethe dyes to be consistent with one another.

K. Base Calls

As discussed above, at each sequencing by synthesis cycle, the signalsthat are observed in the four color channels are used to both determinethe most likely base at that cycle (base call) and to determine aquality score for the base call. Because of a number of factors, it maynot always be the case that the brightest color in the raw dataindicates the most likely base. Thus, it may be desirable to correct forat least one, and more particularly all, of the following phenomena thatwere discussed supra: field flattening, spectral crosstalk, sequencelead and sequence lag. After the correction factors (field flattening,spectral crosstalk and/or lead-lag compensation) have been applied, abase is called based on the maximum signal between the four channels.The output of the base calls may be a file similar to a FASTA format. Inone embodiment, this file is also accompanied by a quality score file.

To optimize the alignment and assembly of the data into contigs, it isdesirable to have a precise quality score associated with each basecall. A quality file may be generated that encodes quality scores foreach cycle. Preserving the information for all four bases is alsodesirable to allow the sequence alignment software to examine severallikely calls instead of only the one with the highest signal.

M. Software Appendices A-C

The below software Appendices A, B and C (copyright IntelligentBio-Systems, Inc., 34 Bear Hill Road, Waltham, Mass. 02451) providesource code for implementing the present invention. Appendix A is asource code for correcting a raw image using a flat map calibrationimage, as exemplified by the code under FlattenlmagelnArray andAdjustRawValue. Appendix B is a source code for applying the inversecross-talk array to four filter images. In particular, theFindBeadIntensities method calls ProcessOneBead to apply the spectralcrosstalk correction matrix. Appendix C is a source code for creating aflat map calibration image. In one embodiment, this is a process thatuses a combination of automated and manual steps. The automated stepsare exemplified by the emoveSpikesWithSlope and LocalSmoothing methods.The manual steps are exemplified by ImageJ to replace spikes in thecalibration image with smoothed data. The manual and automated steps maybe carried out in any order. In a particular embodiment, the manualsteps are carried out before the automated steps.

i. Appendix A

EXPERIMENTAL

The following examples serve to illustrate certain exemplary embodimentsand aspects of the present invention and are not to be construed aslimiting the scope thereof.

Example 1 Materials And Methods

The following is a brief description of the exemplary materials andmethods used in the following Examples. All solvents and reagents werereagent grades, purchased commercially and used without furtherpurification. Protected nucleosides5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine,N⁴-benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxycytidine,N⁶-Benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxyadenosine,N²-isobutyryl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine werepurchased from CNH Technologies, Inc. All other chemicals were purchasedfrom Sigma-Aldrich.

Example 2 Synthesis of 3′-O-Azidomethyl Nucleotides

The synthesis of 3′-O-azidomethyl-dNPTs is described in FIG. 20.Briefly, reaction of 5′-O-TBDMS-2′-deoxynucleosides (5) with a mixtureof DMSO, acetic acid, and acetic anhydride installed the3′-O-methylthiomethyl group (3′-O-MTM, 6), which upon treatment withSO₂Cl₂ converted to activated 3′-O—CH₂Cl (7). The latter can bemonitored in TLC as 3′-OH (5) after dissolving in wet organic solventdue to fast hydrolysis of the —CH₂Cl group. The3′-O—CH₂Cl₂′-deoxynucleoside (7) is then treated with NaN₃ in dry DMFwithout purification to convert to 3′-O—CH₂N₃ (8).3′-O-azidomethyl-2′-deoxynucleosides of A, T, and C (9a-9c) wereobtained in good yield after deprotection of the 5′-O-TBDMS group asdescribed in the FIG. 20. Similar synthesis route for guanosine (G, 9d),lead only very low yield (>10%) due to formation of a number of sidereaction products. To circumvent this, a new method was introduced forthe synthesis of guanosine analog (14) which is described in the FIG.21, which involved protection of the O⁶-group by diphenycarbamoyl group.After protection of this particular group, the intermediate (12-14)became less polar, making easier to purify, and lead good overall yieldin the azidomethyl group installation step.

Example 3 Synthesis of N⁶-benzoyl-3′-O-(azidomethyl)-dA (9a)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Synthesis ofN⁶-Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(6a)

3.0 g N⁶-Benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxyadenosine (5a)(6.38 mmol) was dissolved in a mixture consisting of 11.96 mL DMSO, 5.46mL acetic acid, and 17.55 mL acetic anhydride and stirred at roomtemperature for 48 h. The reaction mixture was then neutralized treatingwith a sufficient amount of saturated NaHCO₃ solution and extracted withCH₂Cl₂ (3×100 mL). The combined organic extract was then washed with asaturated NaHCO₃ solution (100 mL), dried over Na₂SO₄, and concentratedunder vacuum. The resultant yellowish oil was then purified on silicagel column (Hex:EtOAc/1:1 to 1:4) to obtain the productN⁶-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(6a) as white powder in 71% yield (2.4 g, R_(f) 0.6, EtOAc:hex/7:3).HR-MS: obs. m/z 530.2273, calcd. for C₂₅H₃₆O₄N₅SiS 530.2257 [M+H]⁺.¹H-NMR (CDCl₃): δ_(H) 9.00 (s, 1H), 8.83 (s, 1H), 8.35 (s, 1H), 8.05 (d,J=7.6 Hz, 2H), 7.62 (m, 1H), 7.55 (m, 2H), 6.55 (t, J=7.19 Hz, 1H), 4.73(m, 2H), 4.68 (m, 1H), 4.24 (m, 1H), 3.88 (dd, J=11.19, 3.19 Hz, 1H),2.74-2.66 (m, 2H), 2.35 (s, 3H), 0.94 (s, 9H) and 0.13 (s, 6H) ppm.

B. Synthesis of N⁶-benzoyl-3′-O-(azidomethyl)-2′-deoxyadenosine (9a)

To 0.4 gN⁶-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine(0.76 mmol) dissolved in 7 mL dry CH₂Cl₂ was treated with 0.4 mLcyclohexene and 155 μL SO₂Cl₂ (1.91 mmol) at 0° C. for 2 h. During thistime the starting material completely converted to 7a which was shown bydisappearance of the starting material and appearance of 3′-OH analog(5a) in TLC (EtOAC:Hex/7:3, R_(f)˜0.3; the 3-CH₂Cl (7a) could notdetected in TLC due to decomposition in TLC plate to 5a). Then solventwas removed by rotary evaporation and kept about 10 minutes in highvacuum pump. Then dissolved in 5 mL dry DMF and treated with 400 mg NaN₃(6.6 mmol) at room temperature for 3 h. Then the reaction mixture waspartitioned in H₂O/CH₂Cl₂, the combined organic part was dried overNa₂SO₄ and concentrated by rotary evaporation. The crude sample was thendissolved in 5 mL MeOH and treated with 300 mg NH₄F (8.1 mmol) more than38 h. Then MeOH was removed by rotary evaporation. After partioning inH₂O/EtOAc, the combined organic part was dried over Na₂SO₄,concentrated, and purified by silica gel column chromatography (100%EtOAc to 98:2, EtOAc/MeOH) resulting 150 mg of 9a as white powder (48%yield in three steps). HR-MS: Obs. m/z 411.1530, calcd for C₁₈H₁₉O₄N₈411.1529 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.84 (brs, 1H), 8.70 (brs, 1H),8.08 (m, 1H), 7.76-7.54 (m, 5H), 6.47 (t, J=5.6 Hz, 1H), 4.83 (m, 2H),4.78 (m, 1H), 4.39 (m, 1H), 4.09 (d, J=12.78 Hz, H₅′, 1H), 3.88 (d,J=12.78 Hz, H₅″, 1H), 3.09 (m, H₂′, 1H), and 2.65 (m, H₂″, 1H) ppm.

Example 4 Synthesis of 3′-O-azidomethyl-dT (9b)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Preparation of3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b)

2.0 g 5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine (5b) (5.6 mmol)was dissolved in a mixture consisting of 10.5 mL DMSO, 4.8 mL aceticacid, and 15.4 mL acetic anhydride and stirred for 48 h at roomtemperature. The mixture was then quenched by treating with a saturatedNaHCO₃ solution and extracted with EtOAc (3×100 mL). The combinedorganic extract was then washed with a saturated solution of NaHCO₃ anddried over Na₂SO₄, concentrated under vacuum, and finally purified bysilica gel column chromatography (Hex:EtOAc/7:3 to 1:1). The3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b) was obtained as white powder in 75% yield (1.75 g, R_(f)=0.6,hex:EtOAc/1:1). HR-MS: Obs. m/z 417.1890, cald. for C₁₈H₃₃N₂O₅SSi417.1879 [M+H]⁺. ¹H-NMR (CDCl₃): δ_(H) 8.16 (s, 1H), 7.48 (s, 1H), 6.28(m, 1H), 4.62 (m, 2H), 4.46 (m, 1H), 4.10 (m, 1H), 3.78-3.90 (m, 2H),2.39 (m, 1H), 2.14, 2.14 (s, 3H), 1.97 (m, 1H), 1.92 (s, 3H), 0.93 (s,9H), and 0.13 (s, 3H) ppm.

B. Preparation of 3′-O-(azidomethyl)-2′-deoxythymidine (9b)

To 1.095 g3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxythymidine(6b) (2.6 mmol) dissolved in 10 mL dry CH₂Cl₂ were added 1.33 mLcyclohexene and 284 μL SO₂Cl₂ (3.5 mmol) at 0° C. and stirred at theice-cold temperature for 1.5 h. Then the flask temperature was broughtto room temperature and transferred to a round bottom flask. Thevolatiles were removed by rotary evaporation followed by high vacuumpump. Then the crude sample was dissolved in 5 mL dry DMF and 926 mgNaN₃ (15.4 mmol) was added to it and stirred for 3 h at roomtemperature. The crude sample was dispersed in 50 mL distilled water andextracted with CH₂Cl₂ (3×50 mL), the organic extracts were combined anddried over Na₂SO₄ and concentrated by rotary evaporation. The crudesample was then dissolved in MeOH (5 mL) and treated with NH₄F (600 mg,16.2 mmol) for 24 h at room temperature. Then reaction mixture wasconcentrated and partitioned between H₂O/CH₂Cl₂ and the combined organicextract was dried over Na₂SO₄, concentrated, and purified the product bysilica gel column chromatography using Hex:EtOAc/1:1 to 2:5 resultingthe final product (9b) as white powders (˜550 mg, 71% yield in threesteps, R_(f)=0.3, Hex:EtOAc/1:1.5). HR-MS: Observed m/z 298.1146, calcdfor C₁₁H₁₆O₅N₅ 298.1151 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.30 (brs, 1H),7.40 (s, 1H), 6.14 (t, J=6.8 Hz, 1H), 4.79-4.70 (m, 2H), 4.50 (m, 1H),4.16 (m, 1H), 4.01-3.84 (m, 2H), 2.45 (m, 2H) and 1.95 (s, 3H) ppm.

Example 5 Synthesis of N⁴-Benzoyl-3′-O-(azidomethyl)-dC (9c)

The following describes exemplary synthesis steps for compounds shown inFIG. 20.

A. Preparation ofN⁴-Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c)

3.5 g N⁴-benzoyl-5′-O-tert-butyldimethylsilyl-2′-deoxycytidine (5c)(7.65 mmol) was dissolved in a mixture consisting of 14.7 mL DMSO, 6.7mL acetic acid, and 21.59 mL acetic anhydride and stirred for 48 h atroom temperature. During this period of time, a complete conversion toproduct was observed by TLC (R_(f)=0.4, EtOAc:hex/10:1). The mixture wasthen neutralized with a saturated NaHCO₃ solution and extracted withCH₂Cl₂ (3×100 mL). The combined organic extract was then washed withsaturated solution of NaHCO₃ and dried over Na₂SO₄, and concentratedunder vacuum. The product was then purified by silica gel columnchromatography (EtOAc:hex/2:1 to 9:1) to obtainN⁴-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c) as white powder in 73% yield (2.9 g, R_(f)=0.6, EtOAc:hex/9:1).HR-MS: obs. m/z 506.2134, cald. for C₂₄H₃₆O₅N₃SiS [M+H]⁺. 506.2145.¹H-NMR (CDCl₃): δ_(H) 8.43 (d, J=7.1 Hz, 1H), 7.93 (m, 2H), 7.64 (m,1H), 7.54 (m, 3H), 6.30 (m, 1H), 4.62 & 4.70 (2Xd, J=11.59 Hz, 2H), 4.50(m, 1H), 4.19 (m, 1H), 3.84 & 3.99 (2Xdd, J=11.59 & 2.79 Hz, 2H), 2.72(m, 1H), 2.21 (m, 1H), 2.14 (s, 3H), 0.99 (s, 9H), and 0.16 (s, 6H) ppm.

B. Preparation of N⁴-Benzoyl-3′-O-(azidomethyl)-2′-deoxycytidine (9c)

To 0.5580 gN⁴-benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine(6c) (1.04 mmol) dissolved in 8 mL dry CH₂Cl₂ were added 0.56 mLcyclohaxene and 220 μL SO₂Cl₂ (2.7 mmol) at 0° C. and stirred at theice-cold temperature for 1 h. During this time, the starting materialconverted to the chlorinated product as shown by the 3′-OH (5c) compoundin the TLC. The volatiles were then removed under vacuum and resuspendedin dry DMF (5 mL) and treated with NaN₃ (400 mg, 6.6 mmol) and stirredfor 2 h at room temperature. The sample was then partitioned betweenwater and CH₂Cl₂ and the organic extracts were combined and dried overNa₂SO₄ and concentrated under vacuum. The crude sample was thendissolved in MeOH (5 mL) and treated with NH₄F (600 mg, 16.2 mmol) for20 h at room temperature. Then solvent was removed under vacuum andextracted with CH₂Cl₂ and the organic extract was then dried over Na₂SO₄and concentrated under vacuum. The sample was then purified by silicagel column chromatography (Hex:EtOAc 1:4 to 1:10), and the product (9c)was obtained as white powdery substance (˜200 mg, 50% yield in threesteps, R_(f)=0.5, EtOAc:Hex/5:0.5). HR-MS: Obs. m/z 387.1408, calcd forC₁₇H₁₉O₅N₆ 387.1417 [M+H]⁺. ¹H-NMR (CDC₃): δ_(H) 8.30 (d, J=7.2 Hz, 1H),7.93 (d, J=7.50 Hz, 1H), 7.66-7.51 (m, 5H), 6.18 (t, J=6.4 Hz, 1H),4.81-4.68 (m, 2H), 4.52 (m, 1H), 4.25 (m, 1H), 4.08-3.88 (m, 2H), 2.69(m, 1H), and 2.50 (m, 2H) ppm.

Example 6 Synthesis ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-azidomethyl-dG (14)

The following describes exemplary synthesis steps for compounds shown inFIG. 21.

A. Preparation ofN²-isobutyryl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11)

5 g of N²-isobutyryl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11.0 mmol) dissolved in 21 mL dry DMSO was treated with 10 mL aceticacid and 32 mL acetic anhydride, and stirred for 48 h at roomtemperature. The crude reaction mixture was then neutralized by adding aK₂CO₃ solution, and extracted with ethyl acetate (100×3 mL). Thecombined organic extract was then washed with saturated NaHCO₃ solution,dried over Na₂SO₄ and concentrated under vacuum. Then reaction mixturewas purified by a silica gel column chromatography resulting the product11 as white powder (3.9 g, 69% yield; R_(f)=0.35, CH₂Cl₂:MeOH/20:1).HR-MS: Obs. m/z 512.2344 cald. for C₂₂H₃₈O₅N₅SiS 512.2363 [M+H]⁺. ¹H-NMR(CDCl₃): δ_(H) 12.0 (s, 1H), 8.95 (brs, 1H), 8.09 (s, 1H), 6.24 (t,J=6.8 Hz, 1H), 4.73 (m, 2H), 4.66 (m, 1H), 4.16 (m, 1H), 3.81 (m, 2H),2.76 (m, 1H), 2.59 (m, 1H), 2.54 (m, 1H), 2.21 (s, 3H), 1.29 (m, 6H),0.91 (s, 9H), and 0.10 (s, 6H) ppm.

B. Synthesis ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(12)

To 1.0 gN²-isobutyryl-3′-O-(methylthimethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine(11, 1.95 mmol) dissolved in 22 mL dry pyridine were addeddiphenylcarbamoyl chloride (0.677 g, 2.92 mmol) and 1.02 mLN,N-diisopropylethylamine, and stirred at room temperature for 3 h undernitrogen atmosphere. The reaction mixture became dark red during thistime. The solvent was removed under high vacuum, and product was thenpurified by silica gel column chromatography using EtOAc:hex/1:1 to 7:3as mobile phase. The product 12 was isolated as yellowish powder (1.09g, ˜80% yield; R_(f)=0.7, EtOAc:hex (1:1)). HR-MS: Obs. m/z 707.3068calcd. for C₃₅H₄₇O₆N₆SiS 707.3047 [M+H]⁺. ¹H-NMR (CDCl₃): δ_(H) 8.25 (s,1H), 7.94 (brs, 1H), 7.47-7.37 (m, 10H), 6.42 (m, 1H), 4.75 (m, 2H),4.71 (m, 1H), 4.18 (m, 1H), 3.88-3.70 (m, 2H), 2.80 (m, 1H), 2.60 (m,1H), 2.19 (s, 3H), 1.30 (d, J=7.2 Hz, 6H), 0.93 (s, 9H) and 0.14 (s, 6H)ppm.

C. Preparation ofN²-isobutyryl-O⁶-diphenylcarbamoyl-3′-O-azidomethyl-2′-deoxyguanosine(14)

To 786 mg 12 (1.1 mmol) dissolved in 8 mL dry CH₂Cl₂ was treated with0.56 mL cyclohexene and 180 μL SO₂Cl₂ (2.2 mmol) at 0° C. and stirredfor 1.5 h at the same temperature. The solvent was then removed byrotary evaporation, and further dried under high vacuum for 10 minutes.The crude product was then dissolved in 5 mL dry DMF and reacted with600 mg NaN₃ (10 mmol) at 0° C. and stirred at room temperature for 3 h.Reaction mixture was then partitioned H₂O/CH₂Cl₂, the combined organicextract was then dried over Na₂SO₄, and concentrated by rotaryevaporation. The crude was then dissolved in 5 mL dry MeOH, treated with500 mg NH₄F (13.5 mmol) at room temperature for more than 24 h. ThenMeOH solvent was removed by rotary evaporation, and partitioned(H₂O/CH₂Cl₂). The combined organic part was dried over Na₂SO₄ andconcentrated by rotary evaporation and purified by silica gel columnchromatography resulting pure product of 14 as white powder (230 mg,˜36% yield in three steps; hex:EtOAc 1:1 to 1:5, (R_(f)=˜0.3,Hex:EtOAc/1:4). HR-MS: Obs. m/z 588.2343, calcd for C₂₈H₃₀O₆N₉ 588.2319[M+H]⁺. ¹H-NMR (DFM-d₆): δ_(H) 8.64 (brs, 1H), 7.48-7.34 (m, 10H), 6.36(t, J=7.0 Hz), 4.93 (m, 2H), 4.76 (m, 1H), 4.04 (m, 1H), 3.57 (m, 1H),3.34 (m, 2H), 2.97 (m, 1H), 2.81 (m, 1H), and 1.10 (m, 6H).

Example 7 General Method for the Preparation of 3′-O-Azidomethyl-Dntps

The protected 3′-O-azidomethyl nucleoside (0.3 mmol) and proton sponge(75.8 mg; 0.35 mmol) were dried in a vacuum desiccator over P₂O₅overnight before dissolving in trimethyl phosphate (0.60 mL). Thenfreshly distilled POCl₃ (33 μL, 0.35 mmol) was added drop-wise at 0° C.and the mixture was stirred at 0° C. for 2 h. Subsequently, awell-vortexed mixture of tributylammonium pyrophosphate (552 mg) andtributylamine (0.55 mL; 2.31 mmol) in anhydrous DMF (2.33 mL) was addedin one potion at room temperature and stirred for 30 min. Triethylammonium bicarbonate solution (TEAB) (0.1 M, 15 mL, pH 8.0) was thenadded and the mixture was stirred for 1 h at room temperature. Then 15mL of NH₄OH was added and stirred overnight at room temperature. Theresulting mixture was concentrated in vacuo and the residue was dilutedwith 5 mL of water. The crude mixture was then purified with anionexchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradientof TEAB (pH 8.0; 0.1-1.0 M). Further purification by RP HPLC to givecorresponding target as colorless syrup:

Example 8 3′-O-Azidomethyl Nucleotides Cleavage

The 3′-O-azidomethyl group cleavage can be accomplished with a varietyof reducing agents such as phosphines. The cleavage agents that areparticularly desirable are those that are soluble in aqueous media anddo not cause any damage to the DNA. One particularly desirable agent istri(carboethoxy)phosphine (TCEP).

The 3′-O-azidomethyl nucleotides can be separated from nativenucleotides using RP HPLC. In the next experiment, the kinetics of the3′-O-azidomethyl TTP cleavage was studied. For this purpose, a 1 mMsolution of nucleotide was prepared in water and mixed with 50 mMsolution of TCEP/400 mM of Tris at pH 8.0 and incubated at 55 deg C. forvarious periods of time. After the incubation, the reaction was stoppedby mixing with 4 M NaOAc at pH=4.3 and an aliquot of reaction mixture(0.5 nmole of nucleotide) was injected and separated on the RP HPLCcolumn. The integrated peak area was then plotted against time.

Example 9 Sequencing by Synthesis Using 3%0-Azidomethyl Nucleotides

We established conditions for sequencing by synthesis on the surfaceusing 3′-O-azidomethyl nucleotides. For this purpose we used variants ofthe 9 deg N polymerase that were developed specifically to incorporate3′-O-azidomethyl nucleotides. For these sequencing experiments we wereusing synthetic DNA templates that encompass self priming moieties.Examples of these DNA templates and their secondary structures are shownin FIG. 22.

These oligonucleotides carry a 5′-amino modification through which theyare attached to assay surface. The surface constitutes any surface thatis bio-compatible, has low fluorescent background and has functionalgroups on the surface which can be used to covalently attach the DNA. Inthe described case, pre-activated Codelink (from GE Healthcare) slideswere used for this purpose. The solution of the oligonucleotides (50 uM)for spotting was prepared in 150 mM phosphate/bicarbonate spottingbuffer (pH=7.5). The arrays were then spotted and incubated in the humidchamber at 25 deg C. overnight. After the incubation, the arrays wereblocked by washing in the 1×TBST/2% BSA buffer, rinsed with nucleasefree water and dried.

The sequencing was performed in a chambered slide (Grace Biolabs). Inthe experiment, each well was subjected to different number of cyclesusing the mixture of 3′-O-azidomethyl nucleotides with each extensioncycle followed by a cleavage cycle. Extension cycle consisted ofincubating the well with the solution containing: 3′-O-azidomethylnucleotide mix—75 uM, 9 deg N polymerase mutant—250 ug/ml, 10 mM KCl, 10mM (NH4)2SO4, 20 mM Tris-HCl, 4 mM MnSO4, 0.1% Triton-X-100, 0.1%acetylated BSA, pH 8.8. The incubation was carried out at 65 deg C. for20 minutes. After the incubation the wells were washed with Thermopol IIbuffer 3× and then subjected to cleavage with TCEP (100 mM) in 400 mMTris-HCl (pH=8.5) at 65 deg C. for 15 minutes. After the cleavage thewells were washed with the extension reaction buffer (3×) and subjectedto the next extension reaction. The wells were read out with finalreadout mixture consisting of: 2,3′-dideoxynucleotide mix (labeled)—2μM, Therminator II polymerase—250 μg/ml, 10 mM KCl, 10 mM (NH4)2SO4, 20mM Tris-HCl, 2 mM MnSO4, 0.1% Triton-X-10. The structures of thesenucleotides are presented in FIG. 23. After labeling cycle the slide waswashed with wash/block buffer (5×SSC, 0.1% Tween, 2% BSA), rinsed withwater and dried before imaging. Each well was imaged using a prototypesequencing instrument and bases were then called based on the relativeintensity of the observed signal. The result of the experiment ispresented in FIG. 24.

Example 10 Synthesis of 2′-Fluoro, 3′-O-Azidomethyl Nucleotides

The synthesis of 2′-fluoro-3′-O-azidomethyl-dNTPs is described in FIG.25. Briefly, reaction of 5′-O-TBDMS-2′-fluoro-2′-deoxynucleosides (1)with a mixture of DMSO, acetic acid, and acetic anhydride installed the3′-O-methylthiomethyl group (3′-O-MTM, 2), which upon treatment withSO₂Cl₂ converted to activated 3′-O—CH₂Cl (3). The2′-fluoro-3′-O—CH₂Cl₂′-deoxynucleoside (3) is then treated with NaN₃ indry DMF without purification to convert the 3′-O—CH₂Cl to 3′-O—CH₂N₃(4). 2′-fluoro-3′-O-azidomethyl-2′-deoxynucleosides of A, T, and C(5a-5c) can be obtained in good yield after deprotection of the5′-O-TBDMS group as described in FIG. 25. In case of2′-fluoro-3′-O-azidomethyl-2′-deoxybuanosine (G, 5d), the O⁶-group isprotected by diphenycarbamoyl group to increase yield. Finally, therespective nucleosides are phosphorylated using phosphorous oxychloridefollowed by tetrabutylammonium pyrophosphate in the presence of protonsponge (1,8-dimethylaminonaphthalene) and converted to their respectivetriphosphates (6).

Example 11 Synthesis of 2′-Fluoro, 3′-O-Azidomethyl PropargylaminoNucleotides

Synthesis of 2′-fluoro-3′-O-azidomethyl-(propargylamino)-dNTPs isdescribed in FIG. 26. Briefly, reaction of5′-O-TBDMS-2′-fluoro-(5/7-iodo*)-2′-deoxynucleosides (1) with a mixtureof N-trifluoroacetyl-propargylamine, tetrakis(triphenylphosphine)palladium (0) and CuI resulted in the formation of 5/7-propargylamidosubstituted nucleosides (2). In the next step the mixture of DMSO,acetic acid, and acetic anhydride installed the 3′-O-methylthiomethylgroup (3′-O-MTM, 3), which upon treatment with SO₂Cl₂ converted toactivated 3′-O—CH₂Cl (4). The2′-fluoro-3′-O—CH₂Cl-5/7-propargylamido-2′-deoxynucleosides (4) werethen treated with NaN₃ in dry DMF without purification to convert the3′-O—CH₂Cl to 3′-O—CH₂N₃. (5)2′-fluoro-3′-O-azidomethyl-(propargylamino)-2′-deoxynucleosides of A, T,and C (5a-5c) can be obtained in good yield after deprotection of the5′-O-TBDMS group as described in FIG. 26. In case of2′-fluoro-3′-O-azidomethyl-2′-deoxybuanosine (G, 5d), the O⁶-group isprotected by diphenycarbamoyl group to increase yield. *5-iodo,2′-fluoro-2′-deoxy purines and 7-iodo-7-deaza-2′-fluoro-2′-deoxypyrimidines were used as starting material. The synthesis of thesecompounds is well known to those skilled in the art.

Example 12 Spectral Crosstalk Calibration

We calibrated the four color detection system of the above describedexemplary SBS device using a chip spotted with four separate dyes, onein each of four spots. We then made measurements of the chip in all fourchannels, calculated the spectral crosstalk factors and constructed theK and K⁻¹ matrices. FIG. 32 shows the effect of applying the spectralcrosstalk calibration matrix K−1 to raw sequencing data. The datademonstrates that the second base in the sequence would be miscalled asgreen were the spectral crosstalk calibration not performed.

Example 13 Re-Phasing Sequencing by Synthesis Data

As discussed above, dephasing of sequence data is cumulative and canpotentially be significant with longer read lengths. We applied thelead/lag compensation described above to both a 16-base and 25-basesequences containing an AGCT repeat. The results are shown in 33 and 34.FIG. 33A shows the original data captured by the fluorescent detectionsystem and FIG. 33B shows the data after being multiplied by thelead/lag compensation matrix with a lead parameter of 4.5% and a lagparameter of 1%. The relatively high lead parameter was probably due tonative nucleotide contamination in the polymerase preparation. FIG. 33illustrates how the compensation helps to correct miscalls. For example,bases at locations 11, 13 and 15 would be a miscall in the originaldephased data, but are correctly called (as are all the other bases) inthe rephased data. For the 25-base read in FIG. 34, the lead and lagparameters were 1.2% and 1.5% respectively. Although the lead and lagfor this sample were not large enough to create miscalls in the originaldata (FIG. 34A), the lead/lag correction does make the correct base astronger signal compared to the other colors (FIG. 34B). While in bothcorrected sequences (FIGS. 33B and 27B), the matrix multiplicationproduces some negative values, these are probably due to noise, and maybe ignored as long as they are small values. FIG. 34 shows that we wereable to generate data with high fidelity out to 25 bases.

Example 14 Sequencing by Synthesis Data: Extra Washing

In this example, additional washing was done in an attempt to completelyremove the cleaving agent prior to the next cycle in sequencing bysynthesis. Interestingly, increased washing cycles after cleavage stephave only minimal effect on the sequencing performance, as illustratedin the Table below.

Rephased Data 35nt All Templates 25nt Templates Templates % CorrectWashes Lead Lag Lead Lag Calls Qa* 24 2.56% 1.69% 1.75% 1.12% 92.0%0.822 48 1.55% 2.32% 1.20% 1.80% 96.0% 0.862 100 1.40% 2.80% 0.95% 1.65%95.5% 0.826 *Qa = Intensity of the correct base signal/intensity of thesecond highest signalThe metric used to measure the dephasing process is the lead percentagederived empirically to compensate for the lead observed in the run. Onlyat very high wash cycles (i.e. too many washes to be practical) can oneimprove the base calling accuracy.

Example 15 Sequencing by Synthesis Data: Using a Scavenger

In this example, scavengers were used in an attempt to inhibit anyremaining cleaving agent prior to the next cycle in sequencing bysynthesis. As noted above, such compounds can be included in thesolutions used for sequencing by synthesis (or in a separate additionalsolution if desired). In this example, the suitable operatingconcentration for the scavenger in the Extend A/B solutions wasexplored. Two different scavengers were used.

A. Cystamine Scavenger

3′-O-azidomethyl nucleotides labeled with dyes on a cleavable disulfidelinker were used. A range of scavenger concentrations were tested todetermine which concentration is acceptable by the polymerase. The tablebelow shows lead and lag values, and percentage of correct calls for the3′-O-azidomethy/disulfide chemistry in the absence and in the presenceof a first scavenger (cystamine @ 1 mM).

AVG Lead [%] AVG Lag [%] Correct calls [%] NO SCAVENGER 2.0 3.1 93.7CYSTAMINE 1.1 1.9 98.7 SCAVENGERIt is clear from the data in the table that the use of a scavenger canimprove the accuracy of base calling and reduce lead and lag.Importantly, extension reactions performed in the absence and in thepresence of this disulfide based scavenger, cystamine, showed theadditive does not significantly interfere with the extension reaction(FIG. 38).

B. ATA Scavenger

A second scavenger was also tested, i.e. the azido based scavenger, ATA:(11-Azido-3,6,9-trioxaundecan-1-amine). Extension reactions performed inthe absence and in the presence of this azido based scavenger.Nucleotides with 3′-O-azidomethyl groups and with azido based cleavablelinkers were used. The results (FIG. 39) show that the additive does notsignificantly interfere with the extension reaction.

Example 16 Synthesis of Disulfide-Dye Labeled 3′-O-AzidomethylNucleotide

In this example, a method is described for synthesizing a nucleotideanalogue containing an azidomethyl group on the 3″-OH and a labelattached via a disulfide linker (which is cleavable). The scheme isshown in FIG. 40. Preparation of the linker buffer solution: 11 mg of3-((2-aminoethyl)dithio) propionic acid hydrochloride (Prod # 22101 fromPierce Biotech company, 2) was dissolved in 100 μl of 0.1 M sodiumbicarbonate and 900 μl of acetonitrile. 14 μl of triethylamine wasadded. To a solution of 6-carboxy-X-rhodamine, succinimidyl ester (6-ROXSE, cat. #C6126, Invitrogen, 1) (158 μL, 25 mM, 3.96 μmol) in DMF wasadded the above linker solution (500 μl, 50 mM, 25.0 μmol). The reactionmixture was stirred overnight at room temperature and then 800 μl ofTEAB buffer (50 mM, pH 8) was added. The mixture was purified by HPLCand concentrated to give 0.51 μmol of product 3. HPLC method: A, 50 mMtriethylammonium bicarbonate (TEAB) buffer, pH=8.0; B, acetonitrile andeluted with a linear gradient of 0-70% B over 35 minutes and at a flowrate of 2 ml/min. The column used was NoaPak C18, 8×100 mm. Retentiontime for product is 20.5 min. Retention time for hydrolysis of startingmaterial is 18.2 min.

To the above linker-dye conjugate product 3 (0.51 μmol) in 300 μl of DMFwas added a solution of 2,6-dimethylaminopyridine (DMAP) (25 mM, 31 μl,0.77 μmol) and a solution of N,N′-disuccinimidyl carbonate (DSC) (25 mM,31 μl, 0.77 μmol). The reaction mixture was stirred for one hour at roomtemperature. 7-propargylamino, 3′-O-azidomethyl-dATP 5 (1.5 μmol) wasdissolved in 300 μl of water and 40 μl of tri-n-butylamine was added.All solvents were removed under vacuum and the residue dissolved in 300μl of DMF. This solution was then added to the activated linker-dyeconjugate 4 and the mixture was stirred overnight. The reaction mixturewas diluted with 800 μl of TEAB buffer (50 mM, pH 8), purified by HPLCand concentrated. 198 nmol of product 6 was obtained (Retention time forproduct is 18.5 min).

Example 17 Hot Embossing: Millions to Billions of Beads on Slides orChips

In one embodiment, the present invention contemplates such microspheresor beads disposed at high density into microwells or indentations on asurface. It is not intended that the present invention be limited by thenature of the surface or the method of fabrication. Nonetheless, in oneembodiment, the present invention contemplates methods of fabrication togenerate beads on slides at high density.

In one preferred embodiment, the method relies on the use of a hotembossing technique as schematically shown in FIG. 41. Briefly, theprocess employs a stamp (80) having projections (81) that will createdesired features (83) of desired dimensions when pressed into thepolymer (82). The pressing step (B) is typically done with heat andpressure. Thereafter, the stamp is removed and the polymer is cooled(step C). Finally (step D), microspheres (84) containing biomolecules(85) are loaded into the microwells (86). In another embodiment, themethod relies on the use of injection molding technique.

It is not intended that the present invention be limited by the natureof the polymer used in performing the hot embossing or molding process.A variety of polymers can be used including but not limited to: PMMA(polymethyl methacrylate), COP (cycloolefine polymer), and COC(cycloolefine copolymer). In the case of polymers that lack naturalfunctional groups on the surface these groups can be grafted on thesurface by performing ozonation, oxidation, corona discharge treatment,surface plasma or UV treatment or combination thereof. These fabricationmethods allow one to generate substrates with varying features/wellsdensity. Using standard size microscope slides casted out of PMMA or COPpolymers one can create wells with 20 um, 5 um, and 1 um diameters. Theslides with approximately 5 um features (e.g. between 4.8 um and 5.3 um)contain about 40 million microwells per slide, while the 1 um featureslide contains about 1 billion features per slide. With thebiomolecule-containing microspheres deposited within the microwells, asingle slide with such features permits a variety of high throughput,robust assays (e.g. sequencing by synthesis, hybridization, etc.).Nucleic acid fragments representing a large portion of a genome (e.g.human genome) or even an entire genome can be placed on a single slideor handful of slides, and then assayed sequentially or simultaneously.

Example 18 Sequencing: Changing the Spacer Arm Groups or Charge

When performing sequencing by synthesis process one needs to use labelednucleotides to be able to read the signal. In most cases these labelednucleotides after cleavage result in structures that is not of thenative nucleotide. For example, if one uses only labeled nucleotides theDNA structure after cleavage of the dye looks like one shown in FIG. 42(right side). As can be seen, the spacer arm used to attach the dye tothe base still remains attached.

In some cases the spacer also carries a charge, such as for example whenpropargylamino nucleotides are used. In the case of disulfide bonds whatremains after cleavage is the spacer arm with thiol (SH) group attached.The presence of these spacers and groups may affect the ability of thesequencing polymerases to incorporate the subsequent nucleotide. Oneapproach to minimize or eliminate this undesirable effect is to changethe reactivity of the spacer arm groups or their charge by performing achemical “capping” step, where specific reagent is added to react onlywith groups on the spacer arm. This is shown schematically in FIG. 43.

Example 19 Sequencing by Synthesis Data: Using Labeled and UnlabeledNucleotides

As noted previously, the presence of the linkers, spacers and groups onnucleotides may affect the ability of the sequencing polymerases toincorporate the subsequent nucleotide. One approach to minimize oreliminate this undesirable effect is to reduce the amount of labelednucleotides incorporated in the template. Reducing the amount of labelednucleotides that are incorporated can be accomplished by reducing theconcentration of labeled nucleotides in the extension solution, and/ormixing labeled nucleotides (reversible terminators) with non-labeledreversibly terminating nucleotides. In contrast to labeled nucleotides,non-labeled reversible terminator nucleotides after cleavage convert tonative nucleotide.

The effect of reducing the concentration of labeled nucleotides can bebest observed by measuring the ability of polymerase to incorporate thesubsequent nucleotides efficiently and with high fidelity. This is shownin FIGS. 44, 45 and 46.

When the amount of labeled nucleotides is reduced, this results inreduction of fluorescent signal as shown in FIG. 47 (where only labelednucleotides are used in successive extention reactions). In principleonly the amount of signal necessary to decode the nucleotide isrequired. In addition to changing the ratio of labeled and unlabelednucleotides and optimizing it for particular polymerase, one can alsoadjust the time of extension (e.g. reduce extension times down to 1-2minutes) to gain even better control on the signal/incorporation ratioof labeled nucleotides. This is shown in FIGS. 48 and 49 whereadditional performance improvement is achieved upon reducing extensiontime (to 2 minutes and 1 minute, respectively).

All publications and patents mentioned in the above specification areherein incorporated by reference. Various modifications and variationsof the described methods and system of the invention will be apparent tothose skilled in the art without departing from the scope and spirit ofthe invention. Although the invention has been described in connectionwith specific embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments.Indeed, various modifications of the described modes for carrying outthe invention which are obvious to those skilled in the art and infields related thereto are intended to be within the scope of thefollowing claims.

1. A method for carrying out steps of a nucleic acid sequencingprotocol, at least one step performed first in a cycle, some stepsperformed later in said cycle, said method comprising: a) providing: i)a camera; ii) a plurality of reservoirs, at least one of whichcomprising nucleic acid sequencing reagents, and at least one of whichcomprising wash buffer; iii) first nucleic acids in a first flow celland second nucleic acids in a second flow cell, said first and secondnucleic acids comprising templates to be sequenced in an area to beimaged, said flow cells mounted on iv) a moveable support, said supportconfigured to move said flow cells to v) a plurality of stations, atleast some of which are reagent delivery stations, each reagent deliverystation in fluidic communication with at least one of said reservoirsand associated with at least one step, or portion thereof, of saidnucleic acid sequencing protocol, at least one of said steps comprisingpositioning a flow cell such that said area to be imaged can be imagedby said camera; b) moving said moveable support under conditions suchthat said first flow cell is aligned with and connected to the reagentdelivery station associated with a first step in said sequencingprotocol, whereby said first flow cell is in fluid communication with atleast a first reservoir of said plurality of reservoirs; c) introducingreagent via one of said reservoirs into said first flow cell underconditions such that said reagent contacts said first nucleic acids; andd) moving said moveable support under conditions such that i) said firstflow cell is disconnected with said reagent delivery station associatedwith said first step and aligned with a station associated with a laterstep in said sequencing protocol, and ii) said second flow cell isaligned with and connected to said reagent delivery station associatedwith said first step in said sequencing protocol, whereby said secondflow cell is in fluid communication with at least said first reservoirof said plurality of reservoirs.
 2. The method of claim 1, wherein saidlater step comprises the second step in said sequencing protocol andsaid first flow cell is aligned with and connected to said reagentdelivery station associated with said second step in said sequencingprotocol, whereby said first flow cell is in fluid communication with atleast said second reservoir of said plurality of reservoirs.
 3. Themethod of claim 1, wherein said reagent delivery station associated witha first step in said sequencing protocol also performs the second stepin said sequencing protocol.
 4. The method of claim 2, wherein saidlater step comprises the third step in said sequencing protocol.
 5. Themethod of claim 2, wherein said second step is a wash step wherein saidfirst nucleic acids of said first flow cell is contacted with washbuffer.
 6. The method of claim 1, wherein at least one station is adwell station and not a reagent delivery station.
 7. The method of claim6, wherein said later step comprises pausing at said dwell station. 8.The method of claim 1, wherein one of said reservoirs comprises labelednucleotide analogues comprising a base and a sugar.
 9. The method ofclaim 8, wherein said sugar is an acyclic sugar.
 10. The method of claim8, wherein labeled nucleotide analogues comprise a label attached to thebase of said nucleotide analogues via a cleavable linker.
 11. The methodof claim 10, wherein said labeled nucleotide analogues furthercomprising a cleavable blocking group on the sugar.
 12. The method ofclaim 11, wherein said labeled nucleotides are introduced at step c).13. The method of claim 12, wherein the labeled nucleotide analoguesproduce the signals required for imaging and sequencing.
 14. The methodof claim 12, wherein said reagent is introduced into said first flowcell at step c) in a volume that is less than 50 microliters.
 15. Themethod of claim 14, wherein said reagent is introduced into said firstflow cell at step c) in a volume of approximately 20 microliters orless.
 16. The method of claim 12, further comprising the step e)introducing reagent via said first reservoir into said second flow cellunder conditions such that said reagent contacts said second nucleicacids.
 17. The method of claim 16, further comprising the step f) movingsaid moveable support under conditions that i) said first flow cell isdisconnected with said reagent delivery station associated with saidsecond step and aligned with and connected to the reagent deliverystation associated with a third step in said sequencing protocol,whereby said first flow cell is in fluid communication with at least athird reservoir of said plurality of reservoirs, and ii) said secondflow cell is disconnected with said reagent delivery station associatedwith said first step and aligned with and connected to the reagentdelivery station associated with a second step in said sequencingprotocol, whereby said second flow cell is in fluid communication withat least a second reservoir of said plurality of reservoirs.
 18. Themethod of claim 17, wherein said third step in said sequencing protocolis a wash step wherein said first nucleic acids of said first flow cellare contacted with wash buffer.
 19. The method of claim 17, wherein saidfirst nucleic acids in said first flow cell, are scanned and imaged withsaid camera after step f).
 20. The method of claim 19, wherein saidfirst flow cell is removed from said moveable support prior to saidimaging.
 21. The method of claim 19, wherein an oxygen scavenger isintroduced into said first flow cell before, during or after said firstnucleic acids are scanned and imaged.
 22. The method of claim 19,further comprising moving said carousel under conditions that said firstflow cell is aligned with and connected to the reagent delivery stationassociated with a fourth step in said sequencing protocol, whereby saidfirst flow cell is in fluid communication with at least a fourthreservoir of said plurality of reservoirs.
 23. The method of claim 22,wherein said fourth step in said sequencing protocol is a cleavage stepwherein said first chip of said first flow cell is contacted with buffercomprising a cleaving agent which cleaves said cleavable linker underfirst conditions.
 24. The method of claim 23, wherein said cleavableblocking group is thereafter cleaved under second conditions.
 25. Themethod of claim 24, wherein said first conditions and second conditionsare different.
 26. The method of claim 22, further comprising movingsaid carousel under conditions that said first flow cell is disconnectedwith said reagent delivery station associated with said fourth step andaligned with and connected to the reagent delivery station associatedwith a fifth step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a fifth reservoir of saidplurality of reservoirs.
 27. The method of claim 26 wherein said fifthstep in said sequencing protocol comprising introducing a cleaving agentscavenger to address leftover cleaving agent which might prematurelycleave in the next cycle.
 28. The method of claim 17, wherein said firstnucleic acids are on a first chip in said first flow cell, and said chipis scanned and imaged with said camera after step f).
 29. The method ofclaim 28, wherein said first flow cell is removed from said moveablesupport prior to said imaging.
 30. The method of claim 28, wherein saidfirst flow cell is aligned with and connected to the reagent deliverystation associated with a fourth step in said sequencing protocol priorto said imaging with said camera.
 31. The method of claim 28, whereinsaid second nucleic acids are on a second chip in said second flow cell,and said second chip is scanned and imaged after said first chip in saidfirst flow cell is scanned and imaged.
 32. The method of claim 28,wherein said instrument comprises a second camera, and said second chipin said second flow cell is scanned and imaged with said second cameraat the same time said first chip in said first flow cell is scanned andimaged with said first camera.
 33. The method of claim 28, wherein saidinstrument comprises a second camera, and said first chip in said firstflow cell is simultaneously scanned and imaged with said first andsecond cameras.
 34. The method of claim 19 wherein a third flow cellcomprising third nucleic acids is added to said moveable support beforesaid first flow cell is scanned and imaged with said camera.
 35. Themethod of claim 19 wherein a third flow cell comprising third nucleicacids is present on said moveable support when said first flow cell isscanned and imaged with said camera.
 36. The method of claim 35, whereinsaid third flow cell was not present on said moveable support prior tostep b).
 37. The method of claim 19, wherein a third flow cellcomprising third nucleic acids is added to said moveable support aftersaid first flow cell is scanned and imaged with said camera.
 38. Themethod of claim 31, wherein said area to be imaged on said first andsecond chips is approximately 35 mm×2.5 mm, or less.
 39. The method ofclaim 28, wherein said first chip remains in said first flow cell whileit is imaged.
 40. The method of claim 1, wherein at least a portion ofsaid first and second flow cells is transparent.
 41. The method of claim1, wherein said first flow cell is in a first position on said moveablesupport and said second flow cell is in a second position on saidmoveable support, said first and second positions being in a fixedrelationship on said moveable support.
 42. The method of claim 41,wherein said first flow cell is removed from said first position on saidmoveable support and repositioned in a new position on said moveablesupport, said new position being different from said first or secondposition.
 43. The method of claim 42, wherein said first flow cell isremoved from said first position before or after a first cycle of stepsis completed.
 44. The method of claim 1, wherein said moveable supportis a carousel.
 45. The method of claim 44 wherein said carousel is underthe control of a processor.
 46. The method of claim 45 wherein saidprocessor can be programmed to change the movement of said carousel. 47.The method of claim 17, wherein said first nucleic acids are immobilizedon a plurality of beads in said first flow cell, and said beads arescanned and imaged with said camera after step f).
 48. The method ofclaim 1, wherein said first nucleic acids are from a first subject andsaid second nucleic acids are from a second subject.
 49. The method ofclaim 35, wherein said first nucleic acids are from a first subject,said second nucleic acids are from a second subject, and said thirdnucleic acids are from a third subject.
 50. A system comprising i) aplurality of reservoirs, at least one of which comprises nucleic acidsequencing reagents; ii) first nucleic acids in a first flow cell andiii) second nucleic acids in a second flow cell, said first and secondnucleic acids comprising templates to be sequenced in an area to beimaged, said flow cells mounted on iv) a moveable support, said supportconfigured to move said flow cells to v) a plurality of reagent deliverystations, each station in fluidic communication with at least one ofsaid reservoirs and associated with at least one step, or portionthereof, of said nucleic acid sequencing protocol, said steps of saidprotocol carried out by vi) a programmable processor.
 51. The system ofclaim 50, wherein said first nucleic acids are from a first subject andsaid second nucleic acids are from a second subject.
 52. The system ofclaim 50, further comprising a camera for imaging one or more flowcells.
 53. The system of claim 52, wherein at least one of said stationsis positioned such that said area to be imaged can be imaged by saidcamera.
 54. The system of claim 50, wherein said system is containedwithin the housing of an instrument, said instrument comprising a userinterface, said user interface in communication with said programmableprocessor.
 55. The system of claim 52, further comprising an imagingplatform capable of x/y movement, said imaging platform positioned undersaid camera.
 56. The system of claim 55, further comprising a transfermeans capable of moving at least one of said flow cells from saidmoveable support to said imaging platform.
 57. A method for carrying outsteps of a nucleic acid sequencing protocol, at least one step performedfirst in a cycle, some steps performed later in said cycle, said methodcomprising: a) providing: i) an imaging platform capable of x/ymovement; ii) a camera position above said imaging platform; iii) aplurality of reservoirs, at least one of which comprising nucleic acidsequencing reagents, and at least one of which comprising wash buffer;iv) a first chip in a first flow cell and a second chip in a second flowcell, said first and second chips comprising nucleic acid templates tobe sequenced in an area to be imaged, said flow cells mounted on v) acarousel, said carousel configured to move said flow cells to vi) aplurality of stations, at least some stations comprising reagentdelivery stations in fluidic communication with at least one of saidreservoirs and associated with at least one step, or portion thereof, ofsaid nucleic acid sequencing protocol; and vii) a transfer means capableof moving at least one of said flow cells from said carousel to saidimaging platform, and from said imaging platform to said carousel; b)moving said carousel under conditions such that said first flow cell isaligned with and connected to the reagent delivery station associatedwith a first step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a first reservoir of saidplurality of reservoirs; c) introducing reagent via one of saidreservoirs into said first flow cell under conditions such that saidreagent contacts said nucleic acid templates of said first chip; and d)moving said carousel under conditions such that i) said first flow cellis disconnected with said reagent delivery station associated with saidfirst step and aligned with a station associated with a later step insaid sequencing protocol, and ii) said second flow cell is aligned withand connected to said reagent delivery station associated with saidfirst step in said sequencing protocol, whereby said second flow cell isin fluid communication with at least said first reservoir of saidplurality of reservoirs.
 58. The method of claim 57, wherein said laterstep comprises the second step in said sequencing protocol and saidfirst flow cell is aligned with and connected to said reagent deliverystation associated with said second step in said sequencing protocol,whereby said first flow cell is in fluid communication with at leastsaid second reservoir of said plurality of reservoirs.
 59. The method ofclaim 57, wherein said reagent delivery station associated with a firststep in said sequencing protocol also performs the second step in saidsequencing protocol.
 60. The method of claim 57, wherein said later stepcomprises the third step in said sequencing protocol.
 61. The method ofclaim 58, wherein said second step is a wash step wherein said firstchip of said first flow cell is contacted with wash buffer.
 62. Themethod of claim 57, wherein at least one station is a dwell station andnot a reagent delivery station.
 63. The method of claim 62, wherein saidlater step comprises pausing at said dwell station.
 64. The method ofclaim 57, wherein one of said reservoirs comprises labeled nucleotideanalogues comprising a base and a sugar.
 65. The method of claim 64,wherein said sugar is an acyclic sugar.
 66. The method of claim 64,wherein labeled nucleotide analogues comprise a label attached to thebase of said nucleotide analogues via a cleavable linker.
 67. The methodof claim 66, wherein said labeled nucleotide analogues furthercomprising a cleavable blocking group on the sugar.
 68. The method ofclaim 67, wherein said labeled nucleotides are introduced at step c).69. The method of claim 68, wherein the labeled nucleotide analoguesproduce the signals required for imaging and sequencing.
 70. The methodof claim 69, further comprising the step e) introducing reagent via saidfirst reservoir into said second flow cell under conditions such thatsaid reagent contacts said nucleic acid templates of said second chip.71. The method of claim 70, further comprising the step f) moving saidcarousel under conditions that i) said first flow cell is disconnectedwith said reagent delivery station associated with said second step andaligned with and connected to the reagent delivery station associatedwith a third step in said sequencing protocol, whereby said first flowcell is in fluid communication with at least a third reservoir of saidplurality of reservoirs, and ii) said second flow cell is disconnectedwith said reagent delivery station associated with said first step andaligned with and connected to the reagent delivery station associatedwith a second step in said sequencing protocol, whereby said second flowcell is in fluid communication with at least a second reservoir of saidplurality of reservoirs.
 72. The method of claim 71, wherein said thirdstep in said sequencing protocol is a wash step wherein said first chipof said first flow cell is contacted with wash buffer.
 73. The method ofclaim 71, wherein, after step f), said first chip in said first flowcell is moved to a station that is not in fluid communication with areservoir.
 74. The method of claim 73, wherein said first flow cell istransferred from said station not in fluid communication with areservoir to said imaging platform via said transfer means.
 75. Themethod of claim 74, wherein said flow cell transferred to said imagingplatform is scanned and imaged with said camera.
 76. The method of claim75, wherein an oxygen scavenger is introduced into said first flow cellbefore, during or after said flow cell is scanned and imaged.
 77. Themethod of claim 75, wherein, after said flow cell is imaged, said flowcell is transferred back to said carousel via said transfer means. 78.The method of claim 77, wherein said flow cell is transferred back tothe same station from which it was transferred.
 79. The method of claim77, wherein said flow cell is transferred back to a different station.80. The method of claim 77, further comprising moving said carouselunder conditions that said first flow cell is aligned with and connectedto the reagent delivery station associated with a fourth step in saidsequencing protocol, whereby said first flow cell is in fluidcommunication with at least a fourth reservoir of said plurality ofreservoirs.
 81. The method of claim 80, wherein said fourth step in saidsequencing protocol is a cleavage step wherein said first chip of saidfirst flow cell is contacted with buffer comprising a cleaving agentwhich cleaves said cleavable linker under first conditions.
 82. Themethod of claim 81, wherein said cleavable blocking group is thereaftercleaved under second conditions.
 83. The method of claim 82, whereinsaid first conditions and second conditions are different.
 84. Themethod of claim 81, further comprising moving said carousel underconditions that said first flow cell is disconnected with said reagentdelivery station associated with said fourth step and aligned with andconnected to the reagent delivery station associated with a fifth stepin said sequencing protocol, whereby said first flow cell is in fluidcommunication with at least a fifth reservoir of said plurality ofreservoirs.
 85. The method of claim 84, wherein said fifth step in saidsequencing protocol comprising introducing a cleaving agent scavenger toaddress leftover cleaving agent which might prematurely cleave in thenext cycle.
 86. The method of claim 79, wherein said different stationis adjacent a reagent delivery station associated with a later step inthe same cycle of said sequencing protocol.
 87. The method of claim 79,wherein said different station is the reagent delivery stationassociated with a later step in the same cycle of said sequencingprotocol.
 88. The method of claim 79, wherein said different station isadjacent a reagent delivery station associated with a first step in alater cycle of said sequencing protocol.
 89. The method of claim 79,wherein said different station is the reagent delivery stationassociated with a first step in a later cycle of said sequencingprotocol.
 90. The method of claim 88, further comprising moving saidcarousel under conditions such that said first flow cell is aligned withand connected to said reagent delivery station associated with a firststep in said sequencing protocol, whereby said first flow cell is influid communication with at least a first reservoir of said plurality ofreservoirs, such that a second cycle of said sequencing protocol cancommence.
 91. The method of claim 88, wherein there is at least onedwell station between the station from which the flow cell wastransferred and the station adjacent a reagent delivery stationassociated with a first step in said sequencing protocol.
 92. The methodof claim 57, wherein said first flow cell is in a first position on saidmoveable support and said second flow cell is in a second position onsaid moveable support, said first and second positions being in a fixedrelationship on said moveable support.
 93. The method of claim 92,wherein said first flow cell is removed from said first position on saidmoveable support and repositioned in a new position on said moveablesupport, said new position being different from said first or secondposition.
 94. The method of claim 93, wherein said first flow cell isremoved from said first position before or after a first cycle of stepsis completed.
 95. The method of claim 57, wherein said nucleic acidtemplates in said first flow cell are from a first human patient andsaid nucleic acid templates in said second flow cell are from a secondhuman patient.
 96. A method for carrying out steps of a nucleic acidsequencing protocol, at least one step performed first in a cycle, somesteps performed later in said cycle, said method comprising: a)providing: i) a plurality of flow cells, the number of flow cellscorresponding approximately to the number of steps in one cycle, saidflow cells comprising nucleic acid templates to be sequenced in an areato be imaged, said flow cells mounted on ii) a moveable support, saidsupport configured to move said flow cells to iii) a plurality ofstations, at least some of which are reagent delivery stations, eachreagent delivery station in fluidic communication with at least onereservoir of a plurality of reservoirs and associated with at least onestep, or portion thereof, of said nucleic acid sequencing protocol, someof said reservoirs comprising reagent; b) moving said moveable supportunder conditions such that a first flow cell of said plurality of flowcells is aligned with and connected to the reagent delivery stationassociated with a first step in said sequencing protocol, whereby saidfirst flow cell is in fluid communication with at least a firstreservoir of said plurality of reservoirs; c) introducing reagent viaone of said reservoirs into said first flow cell under conditions suchthat said reagent contacts said nucleic acid templates in said firstflow cell; and d) moving said moveable support under conditions suchthat said first flow cell is disconnected with said reagent deliverystation associated with said first step and aligned with a stationassociated with a later step in said sequencing protocol.
 97. The methodof claim 96, wherein each cycle comprises five steps and the number offlow cells is between five and seven.
 98. The method of claim 96,wherein each cycle comprises six steps and the number of flow cells isbetween four and eight.
 99. The method of claim 96, wherein each cyclecomprises seven steps and the number of flow cells is between five andnine.
 100. The method of claim 96, wherein the number of flow cells isequal to the number of steps in one cycle.
 101. The method of claim 96,wherein said nucleic acid templates are on a first chip in said firstflow cell.
 102. The method of claim 96, wherein said nucleic acidtemplates are immobilized on a plurality of beads in said first flowcell.
 103. The method of claim 96, wherein said area to be imaged isapproximately 35 mm×2.5 mm, or less.
 104. The method of claim 103,wherein said area to be imaged is not less than 10% of the area definedby 35 mm×2.5 mm.
 105. The method of claim 96, wherein said reagent ofstep c) comprises labeled nucleotide analogues, said analoguescomprising i) a label attached to the base of said nucleotide analoguesvia a cleavable linker and ii a cleavable blocking group on the sugar.106. The method of claim 105, further comprising performing a pluralityof cycles, wherein steps b, c and d are repeated.